Updated assertions.go to include `ShouldWrap`

This updates the tests to include some changes from Pull #438.

I believe this confirms that the pull isn't actually needed.

Update parser tests.

Refine third-party lib checking scripts.

This also selects a stable version for smartystreets/assertions.

Set minimum Go version to 1.16

Lots of potential cleanups here; Since I appear to be the only one maintaining 
goconvey at the moment, I'd like to focus on cleaning up a lot of the legacy
code here for now, with the intent on getting `convey` to integrate better
with regular `go test` functionality (like relying on go modules, go:embed, 
sub-tests), and  to have `goconvey` work better with tests _not_ using 
`convey`.

If someone's interested in maintaining goconvey support for older Go
versions, let's talk in this PR.

Use go:embed to serve static resources.

This will help allow `goconvey` to be distributed as a standalone binary.

Remove >go1.2 detection logic.

This keeps things simple and just uses one temporary directory per
`goconvey` process to store all coverage reports, with an optional
flag to leak the temporary directory after shutting down the server.

This will allow `goconvey` to properly report coverage for code
using go modules.

Fixes #618
Fixes #564

Use temporary directory for coverage reports.