Group all ABI tests.

[eddyb]

There are a bunch of run-pass tests that should be in run-pass/abi (or some other directory if anyone has a better suggestion), and many of them are mentioned here:

rust/src/test/run-pass/abi-sysv64-arg-passing.rs

Lines 7 to 23 in baab191

	// cabi-int-widening
	// extern-pass-char
	// extern-pass-u32
	// extern-pass-u64
	// extern-pass-double
	// extern-pass-empty
	// extern-pass-TwoU8s
	// extern-pass-TwoU16s
	// extern-pass-TwoU32s
	// extern-pass-TwoU64s
	// extern-return-TwoU8s
	// extern-return-TwoU16s
	// extern-return-TwoU32s
	// extern-return-TwoU64s
	// foreign-fn-with-byval
	// issue-28676
	// struct-return

You can also look for tests that mention rust_test_helpers - most of the functions from that C library are used to test ABIs.

cc @nagisa @rkruppe

This issue has been assigned to @kper via this comment.

[kper]

@rustbot claim

[kper]

Is there a way to test if it's still working?

[eddyb]

@kper look at the number of tests reported by ./x.py test --stage 1 src/test/run-pass.
They should be identical before and after the move.

[kper]

@eddyb ok, the results look good. Can you check?