make use of cargo workspace for tests

[Be-ing]

There is no need to run cargo test/clippy/fmt for each crate individually in the workspace. Reusing the workspace target directory that Corrosion uses drastically increases the speed of the tests by reusing compilation artifacts from the main build.

Depends on #229 to avoid merge conflicts

[Be-ing]

:/ Windows is failing to link the tests...

  = note: cxx-qt-gen.lib(ffi.cxx.o) : error LNK2019: unresolved external symbol get_cpp_number referenced in function cxxbridge1$get_cpp_number
          C:\cxx-qt\build\cargo\build\debug\deps\basic_cxx_only-c76a0ebb8efe9746.exe : fatal error LNK1120: 1 unresolved externals

[Be-ing]

That link error already exists in the main branch, but it was obscured by excluding most of the tests on Windows.

[ahayzen-kdab]

This was implemented in another change.

[Be-ing]

It was? What change? CMake is still calling cargo test per crate on the main branch.

[ahayzen-kdab]

Oh lol, sorry i thought this was my workspace PR which put all the examples in the same workspace :')

[OlivierLDff]

I will have a look at that if you want ;)

[Be-ing]

@OlivierLDff that would be great :)

[Be-ing]

Superseded by #509