lib: migrate every test file to the canonical *_test.ww name

25 renames (git mv, content untouched). _test.ww is what the package
coordinator's test detection and the sep loader's canonical exclusion
key on; the old *test.ww spellings survived only through the
line-leading-@test compatibility scan. Consumers updated in place:
LIBRARY_TESTS, the libbyteid roster, the 901/974/975/976 carriers that
copy or invoke these files, and the check.c/check.ww + path/ftos
comments that cite them. Closes the open-driver-work migration bullet.

test/wcc/976_stat_run.c

@@ -10,9 +10,9 @@
  *        <tmp>/symlink   →  ./regfile  (relative symlink)
  *        <tmp>/subdir       directory, mode 0700
  *   3. setenv WW_TEST_STAT_REGFILE / SYMLINK / SUBDIR / NOENT with
- *      the absolute paths; lib/os/stattest.ww reads them via
+ *      the absolute paths; lib/os/stat_test.ww reads them via
  *      os.getenv and exercises stat/lstat/fstat/exists.
- *   4. exec `ww test lib/os/stattest.ww`.
+ *   4. exec `ww test lib/os/stat_test.ww`.
  *   5. Remove the exact children and then the mkdtemp-owned root.
  *
  * Same wrapper shape as 970-975; cleanup runs on success and failure.
@@ -62,7 +62,7 @@ main(void)
 	snprintf(symlink_path, sizeof symlink_path, "%s/symlink", tmpl);
 	snprintf(subdir, sizeof subdir, "%s/subdir", tmpl);
 	snprintf(noent, sizeof noent, "%s/does-not-exist", tmpl);
-	const char *src = "lib/os/stattest.ww";
+	const char *src = "lib/os/stat_test.ww";
 	int fail = 0, subdir_owned = 0;
 
 	/* Populate. */