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.

selfhost/cmd/wcc/check.ww

@@ -218,7 +218,7 @@ fn srcimports(file: *syntax.node, modtag: str, name: str) bool = {
 	let u: *syntax.node = file.list;
 	for (u != nil) {
 		if (u.kind == syntax.nkind.N_USE) {
-			// Skip self-imports: lib/fmt/fmttest.ww carries
+			// Skip self-imports: lib/fmt/fmt_test.ww carries
 			// `use fmt;` while its module tag is also "fmt".
 			// That directive doesn't introduce a foreign
 			// module bareword and lib/fmt's own