Fix z/OS archive test failure on macOS (#197290)
Fixes failures introduced by #187110.
- https://lab.llvm.org/buildbot/#/builders/190/builds/42544
- https://lab.llvm.org/buildbot/#/builders/23/builds/19989
-
https://logs.chromium.org/logs/fuchsia/buildbucket/cr-buildbucket/8682002400978909393/+/u/clang/test/stdout
The original test hit a "file to small" error on macOS before hitting
the expected "truncated or malformed archive" error. This patch updates
the test to generate a valid archive then truncates it to 28 bytes so
that its large enough to pass the initial size check but too small for
the 60-byte member header, so it correctly hits the expected failure on
all platforms.
