test_cp.rs: fix error message of ficlone on musl

[oech3]

Closes #14274

[github-actions]

Binary size comparison:

Individual binary size comparison VS main (threshold: >=5% AND >=4 KB).

Total size of compared binaries: 152.18 MB (+1004 KB, +0.65%)

Significant per-binary changes:
  comm     1.12 MB ->    2.33 MB  (+1.21 MB, +107.29%)

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/tail/tail-n0f (passes in this run but fails in the 'main' branch)

[xtqqczze]

Suggested change

	.stderr_contains("ross-device link"); // cover both of glibc and musl
	.stderr_contains(if cfg!(target_env = "musl") {
	"ross-device link"
	} else {
	"Invalid cross-device link"
	});

I'd prefer to be more explicit here.

[oech3]

I don't think there is worth to increase complexity at here just for test.

[xtqqczze]

I think the explicit cfg!(target_env = "musl") is worthwhile here. The test is covering a known difference in the diagnostic, so checking the expected string for each environment makes that clearer. The added complexity is small and limited to the test.

[oech3]

It does not cover the case musl started aligning message with GNU at some version (or inverse).

[xtqqczze]

If the platform message changes, I think it's useful for the test to fail. It flags the change and lets us decide if anything needs updating.

[xtqqczze]

To be honest I think we should split test into separate test_cp_reflink_always_invalid_argument and test_cp_reflink_always_cross_device tests.

[sylvestre]

don't hesitate to submit a new PR :)

[xtqqczze]

#14282