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/975_dirs_run.c

@@ -5,9 +5,9 @@
  * Workflow:
  *   1. mkdtemp /tmp/wwdirs-XXXXXX  — fresh per run, no cross-run
  *      stale state, no parallel-test conflicts.
- *   2. setenv the four cohort env states (see lib/dirs/dirstest.ww
+ *   2. setenv the four cohort env states (see lib/dirs/dirs_test.ww
  *      file header for the contract).
- *   3. exec `ww test lib/dirs/dirstest.ww`. The fixture reads
+ *   3. exec `ww test lib/dirs/dirs_test.ww`. The fixture reads
  *      WW_TEST_TMPDIR + the XDG_/HOME envs and asserts.
  *   4. Remove the exact auto-created child dirs bottom-up, then the
  *      mkdtemp-owned root.
@@ -63,7 +63,7 @@ main(void)
 	unsetenv("XDG_STATE_HOME");
 	setenv("WW_TEST_TMPDIR", tmpl, 1);
 
-	const char *src = "lib/dirs/dirstest.ww";
+	const char *src = "lib/dirs/dirs_test.ww";
 	char path[1024], cmd[2048];
 	snprintf(path, sizeof path, "%s/%s", cwd, src);
 	snprintf(cmd, sizeof cmd, "%s/ww test %s", bin, path);