Move tests into the tests directory & run tests on GH Actions

[sampoder]

The successor to #121... rebasing is doing my head in but full steam ahead! This PR works towards resolving #82 & #60.

[conradev]

We need to fix two things here:

• The test command should match the build command more. We should test all targets and features. We should also make explicit which targets we are testing in the matrix configuratio (i.e. the host target)

• The test command seemingly recompiles the crate, I'd be curious why it can't re-use the previous compile (might be for the above reason)

[sampoder]

Let me look into recompilation, that'll speed things up a lot when removed!

[sampoder]

Suggested change

	run: cargo test
	run: cargo test --verbose --workspace --all-features --target ${{ join(matrix.targets, ' --target ') }}

@conradev is this what you're looking for?

[sampoder]

The test command seemingly recompiles the crate, I'd be curious why it can't re-use the previous compile (might be for the above reason)

Based on sagiegurari/cargo-make#251, it seems like removing cargo build [...] and just doing cargo test [...] would achieve what we'd want.