fix(ci): remove accidentally-committed test_st_pread binary

#421 committed the compiled tests/test_st_pread (mode 644, non-executable).
On a fresh CI checkout make sees it as up-to-date and skips rebuilding, so
run_tests.py hits 'Permission denied' exec'ing a non-executable file — the
linux/macos 'make check' failure. Remove the binary and gitignore it next to
the existing test_st_mirror rule.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

c/.gitignore

@@ -4,3 +4,4 @@ olmoe_hf/
 olmoe_i4/
 .build-config
 tests/test_st_mirror
+tests/test_st_pread