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.

cmd/wcc/check.c

@@ -2900,7 +2900,7 @@ src_imports(Node *file, const char *modtag, const char *name)
 	if (file == NULL || name == NULL || name[0] == '\0') return 0;
 	for (Node *u = file->list; u; u = u->next) {
 		if (u->kind != N_USE) continue;
-		/* Skip self-imports: lib/fmt/fmttest.ww carries `use fmt;`
+		/* Skip self-imports: lib/fmt/fmt_test.ww carries `use fmt;`
 		 * even though its module tag is also "fmt"; that directive
 		 * doesn't introduce a foreign module bareword and lib/fmt's
 		 * own `fn bsprintf(fmt: str, ...)` is not a shadow of it. */