tests/sys/vfs: add ABI-root absolute symlink tests

[dteske]

Summary

Pin the overlay namei(9) uses for Linux ABI processes: a target under
the ABI root wins (PR 289739), a native-only target should retry from
the native root (PR 297426), an ENOENT past a resolved ABI target is
not retried, and the plain no-symlink native fallback is unchanged.
Skip without a Linux userland so CI stays green. The two native-only
cases use atf_expect_fail until the retry lands.

PR: 297426

[kib]

I did not read the details of the tests, but I do not see a reason not to commit them now.

Still I have one suggestion (or request if you prefer that name). Could you please annotate each test with expanded file structures that describe the disposition of the directories and symlinks created by tests. I mean, I might want to debug the stuff instead of just running tests, and for that I want to be able to recreate the test configuration of files without running the kyua machinery. Then not having to decipher the innards of the test shell scripts would save energy.

[dteske]

In D59990#1376636, @kib wrote:

I did not read the details of the tests, but I do not see a reason not to commit them now.

Still I have one suggestion (or request if you prefer that name). Could you please annotate each test with expanded file structures that describe the disposition of the directories and symlinks created by tests. I mean, I might want to debug the stuff instead of just running tests, and for that I want to be able to recreate the test configuration of files without running the kyua machinery. Then not having to decipher the innards of the test shell scripts would save energy.

Will do. Tree comment on each case: native vs ABI layout, the symlink, and the expected result, so the fixture can be rebuilt by hand without kyua.

[dteske]

Add reproduction step comments to tests