test: stop four fixtures asserting Unix-shaped assumptions - #562

Four of the twelve known windows failures were fixtures encoding assumptions that hold only on Unix. None is a
product defect — each asserts something true in one place and meaningless in the other.

test_compensation.star — the failure never happened

It forced the copy to fail by targeting /dev/null/impossible/path.txt, which fails on Unix because /dev/null
exists and is not a directory. On Windows that names an ordinary path, so the copy succeeded, compensation
never ran, and the test was quietly creating directories at the root of the current drive.

It now fails the copy the way gather_api_test.go already does — a destination whose parent is a regular file,
which no platform will make a directory over.

test_shell_exec.star and test_source.star — sh ate the path

Both interpolate t.tmp(...) into an sh -c command string. t.tmp returns filepath.Join(...), so on Windows
that path is native and backslash is sh's escape character:

C:\Users\RUNNER~1\AppData\...   →   C:UsersRUNNER~1AppData...

a drive-relative path, so the redirect wrote somewhere unintended and expect_file reported it missing.

Single-quoting the destination fixes both. POSIX single quotes preserve backslashes literally, and the MSYS
runtime forwards a Windows path to the Win32 API unconverted. Path translation was never involved — it
applies to arguments handed to native executables, and a redirect is resolved by sh itself.

The same hazard exists in production: pkg/platform concatenates package names into shell strings and runs some
under sudo. Filed as #561, where the remedy is argv rather than better quoting.

TestGatherFailureUnwind_ViaPublicAPI — the error names the parent

It asserted the run error contains boom.txt. The write fails while creating boom's parent, and the two
platforms name different halves of that failure — Windows reports mkdirat blocker: file exists.

The assertion now looks for blocker, which both platforms name and which only the boom item has, so it
identifies the failing iteration just as precisely. Verified against the Unix error before changing it.

Verified

• make check — clean
• Baseline expected 12 → 8, verified name-for-name before merge

Refs #373.