stdbuf: support libstdbuf in same directory as stdbuf

[Ecordonnier]

Make stdbuf search for libstdbuf first in the directory where stdbuf is running. This matches GNU coreutils behavior. Add a symlink to deps/libstdbuf in order to make it easier to run the stdbuf tests with feat_external_stdbuf enabled.

Remove tests which are assuming that libstdbuf is not found. Those tests are now always failing, because the build directory contains a symlink to deps/libstdbuf and libstdbuf is always found.

Fixes #10345

[oech3]

Unfortunately, Cygwin (windows) does not allow symlink by default. Would you disable tests on Cygwin?

[codspeed-hq]

CodSpeed Performance Report

Merging this PR will not alter performance

_{Comparing Ecordonnier:eco/libstdbuf-same-dir (7bd4c53) with main (5cb884f)}

Summary

✅ 142 untouched benchmarks
⏩ 180 skipped benchmarks¹

Footnotes

1. 180 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[github-actions]

GNU testsuite comparison:

Note: The gnu test tests/cut/cut-huge-range is now being skipped but was previously passing.

[Ecordonnier]

Unfortunately, Cygwin (windows) does not allow symlink by default. Would you disable tests on Cygwin?

Which test is failing on Cygwin? test_stdbuf_search_order_exe_dir_first ?

[oech3]

Cygwin package does not run test currently, but CI would not allow all of tests creating symlink.

[Ecordonnier]

Cygwin package does not run test currently, but CI would not allow all of tests creating symlink.

The test I am adding does not create a symlink, so it is not clear to me what you are asking me to do.

[oech3]

Add a symlink to deps/libstdbuf in order to make it easier to run the stdbuf tests with feat_external_stdbuf enabled.

Does it means tester should manually create a symlink?

[Ecordonnier]

Add a symlink to deps/libstdbuf in order to make it easier to run the stdbuf tests with feat_external_stdbuf enabled.

Does it means tester should manually create a symlink?

The build system creates a symlink, and would print a warning on cygwin if the command to create the symlink fails.

[oech3]

would print a warning on cygwin if the command to create the symlink fails.

Thats fine for me!

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tty/tty-eof (fails in this run but passes in the 'main' branch)