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.

lib/strconv/test/ftos_test.ww

@@ -1,5 +1,5 @@
 // ftostest — exercises lib/strconv/ftos.ww (Hare ftos.ha / ftos_ryu.ha
-// Ryū port). Run with `out/bin/ww run lib/strconv/test/ftostest.ww`.
+// Ryū port). Run with `out/bin/ww run lib/strconv/test/ftos_test.ww`.
 // A failing row aborts via the assert/abort builtin (task #5 @test
 // conversion).
 //