Add ui test of malformed attribute containing expression

Update to syn 2

Rewrite attribute parser using parse_nested_meta

    error: you seem to be trying to use `match` for destructuring a single pattern. Consider using `if let`
        --> serde_derive_internals/src/attr.rs:1412:8
         |
    1412 |       Ok(match string.parse() {
         |  ________^
    1413 | |         Ok(path) => Some(path),
    1414 | |         Err(_) => {
    1415 | |             cx.error_spanned_by(
    ...    |
    1420 | |         }
    1421 | |     })
         | |_____^
         |
         = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#single_match_else
         = note: `-D clippy::single-match-else` implied by `-D clippy::pedantic`
    help: try this
         |
    1412 ~     Ok(if let Ok(path) = string.parse() { Some(path) } else {
    1413 +         cx.error_spanned_by(
    1414 +             &string,
    1415 +             format!("failed to parse path: {:?}", string.value()),
    1416 +         );
    1417 +         None
    1418 ~     })
         |

    error: you seem to be trying to use `match` for destructuring a single pattern. Consider using `if let`
        --> serde_derive_internals/src/attr.rs:1434:8
         |
    1434 |       Ok(match string.parse() {
         |  ________^
    1435 | |         Ok(expr) => Some(expr),
    1436 | |         Err(_) => {
    1437 | |             cx.error_spanned_by(
    ...    |
    1442 | |         }
    1443 | |     })
         | |_____^
         |
         = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#single_match_else
    help: try this
         |
    1434 ~     Ok(if let Ok(expr) = string.parse() { Some(expr) } else {
    1435 +         cx.error_spanned_by(
    1436 +             &string,
    1437 +             format!("failed to parse path: {:?}", string.value()),
    1438 +         );
    1439 +         None
    1440 ~     })
         |

    error: you seem to be trying to use `match` for destructuring a single pattern. Consider using `if let`
        --> serde_derive_internals/src/attr.rs:1478:8
         |
    1478 |       Ok(match string.parse() {
         |  ________^
    1479 | |         Ok(ty) => Some(ty),
    1480 | |         Err(_) => {
    1481 | |             cx.error_spanned_by(
    ...    |
    1486 | |         }
    1487 | |     })
         | |_____^
         |
         = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#single_match_else
    help: try this
         |
    1478 ~     Ok(if let Ok(ty) = string.parse() { Some(ty) } else {
    1479 +         cx.error_spanned_by(
    1480 +             &string,
    1481 +             format!("failed to parse type: {} = {:?}", attr_name, string.value()),
    1482 +         );
    1483 +         None
    1484 ~     })
         |

…on metadata

Check for None-delimited group in attribute value

Fixes these being treated as "tests" by `cargo test` in serde_derive:

    running 3 tests
    test src/internals/check.rs - internals::check::check_remote_generic (line 23) ... FAILED
    test src/internals/check.rs - internals::check::check_remote_generic (line 29) ... FAILED
    test src/lib.rs - (line 3) ... ok

    failures:

    ---- src/internals/check.rs - internals::check::check_remote_generic (line 23) stdout ----
    error: unknown start of token: \u{2026}
     --> src/internals/check.rs:25:20
      |
    4 | struct Generic<T> {…}
      |                    ^

    error: cannot find attribute `serde` in this scope
     --> src/internals/check.rs:24:3
      |
    3 | #[serde(remote = "Generic")]
      |   ^^^^^
      |
      = note: `serde` is in scope, but it is a crate, not an attribute

    error[E0392]: parameter `T` is never used
     --> src/internals/check.rs:25:16
      |
    4 | struct Generic<T> {…}
      |                ^ unused parameter
      |
      = help: consider removing `T`, referring to it in a field, or using a marker such as `PhantomData`
      = help: if you intended `T` to be a const parameter, use `const T: usize` instead

    ---- src/internals/check.rs - internals::check::check_remote_generic (line 29) stdout ----
    error: unknown start of token: \u{2026}
     --> src/internals/check.rs:31:21
      |
    4 | struct ConcreteDef {…}
      |                     ^

    error: cannot find attribute `serde` in this scope
     --> src/internals/check.rs:30:3
      |
    3 | #[serde(remote = "Generic<T>")]
      |   ^^^^^
      |
      = note: `serde` is in scope, but it is a crate, not an attribute

Currently fails:

    error: unexpected end of input, unexpected token in nested attribute, expected ident
     --> test_suite/tests/regression/issue2415.rs:4:9
      |
    4 | #[serde()]
      |         ^

Accept empty #[serde()] attribute