When a script has failed, the output of the commands running in the
background can be useful to see, so display it then and also in verbose
mode.

This lets us wait for an individual background command rather
than all of them at once.

There's no need to use it and every dependency lost is good.

Trivia: the first defined Go version build constraint is go1.1.

Otherwise, commands like `go mod tidy` will try to walk into the default
GOPATH directory. At best, that's a waste of resources, since
directories like GOMODPATH may contain lots of files. At worst, it
results in errors, as directories in GOMODPATH are not valid import paths:

	--- FAIL: TestScripts/list (0.01s)
		testscript.go:413:
			[...]
			> go mod tidy
			[stderr]
			mod/gopath/pkg/mod/fruit.com@v1.1.0/fruit: import path "mod/gopath/pkg/mod/fruit.com@v1.1.0/fruit" should not have @Version

We add the test in goproxytest, as goproxytest already uses gotooltest,
and we need an external module to reproduce the error.

While here, bump gotip and the action versions.

flag.ExitOnError will exit 0 for -h/-help, while the flag.PanicOnError
override in runCoverSubcommand was always exiting 2, making the
success/failure of running a command with -hhelp depend on whether
coverage is active.

For backwards compatibility, both testscript and goproxytest, which used
to glob on `*.txt`, now look for both file extensions.
Note that this required a bit of a refactor in testscript,
as we cannot use a single glob expression to accomplish this.

Code which produces files, such as txtar-addmod, now produces `*.txtar`
rather than `*.txt`. Similarly, the public docs now use `*.txtar` too.

Note that we leave many txt files in tests untouched;
it's unnecessary to change them given the backwards compatibility,
and it has zero benefit to the user as they aren't public.
Moreover, the diff churn would make this patch harder to review.
If a future version of go-internal only supports txtar extensions,
then it could replace all of those extensions accordingly.

Fixes #126.

We also document how top-level commands fed to RunMain work with and
without "exec" the same way, and how RequireExplicitExec can drop
backwards compatibility for greater consistency.

Fixes #163.

From Go 1.19, the build constraint 'unix' proposed in golang/go#20322 is
satisfied by any sufficiently Unix-like value of GOOS, as defined by
src/go/build/syslist.go. This commit adds a 'UnixOS' list containing the
values of GOOS that would satisfy the 'unix' constraint in Go 1.19.

Co-authored-by: Paul Jolly <paul@myitcv.io>

From Go 1.19, the build constraint 'unix' proposed in golang/go#20322 is
satisfied by any sufficiently Unix-like value of GOOS, as defined by
src/go/build/syslist.go. This commit adds a predefined 'unix' condition
with the same meaning, available for use in test scripts. The condition
is satisfied if the target GOOS is one of the list of Unix-like systems
defined in 'imports.UnixOS'.

Fixes #166.

Co-authored-by: Daniel Martí <mvdan@mvdan.cc>

They used args[0], which is the first argument to the command,
rather than the command name itself:

	> gofumpt foo.go
	FAIL: [...] use 'exec foo.go' rather than 'foo.go' (because RequireExplicitExec is enabled)

I believe I introduced this regression when refactoring the pull request
due to Roger's review, as I moved the code out of TestMain, where
args[0] was still the name of the command to be run.

Note that I can't easily add a regression test here, because the
testscript command being used in the tests is not a top-level command
run as a separate process, so it does not produce stdout nor stderr.
The change seems easy enough to review manually,
and our tests don't strictly check all error messages anyway.

While here, remove the unnecessary use of -verbose.

They used args[0], which is the first argument to the command,
rather than the command name itself:

	> gofumpt foo.go
	FAIL: [...] use 'exec foo.go' rather than 'foo.go' (because RequireExplicitExec is enabled)

I believe I introduced this regression when refactoring the pull request
due to Roger's review, as I moved the code out of TestMain, where
args[0] was still the name of the command to be run.

Note that I can't easily add a regression test here, because the
testscript command being used in the tests is not a top-level command
run as a separate process, so it does not produce stdout nor stderr.
The change seems easy enough to review manually,
and our tests don't strictly check all error messages anyway.

While here, remove the unnecessary use of -verbose.

We have been setting `/`, `:`, and `$` for some time,
but they weren't documented. Do that.

`goversion` doesn't belong here, as it was moved to `gotooltest`.

`TMPDIR` was slightly wrong; it has started with a period for some time,
to ensure that tools like `go list ./...` don't descend into it.