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/path/path_test.ww

@@ -1,5 +1,5 @@
 // pathtest — exercises lib/path (c2-stack subset). Run with
-// `out/bin/ww run lib/path/pathtest.ww`. A failing row aborts via the
+// `out/bin/ww run lib/path/path_test.ww`. A failing row aborts via the
 // assert/abort builtin (task #5 @test conversion).
 //
 // Vectors mirror Hare's @test fns in ref/hare/path/stack.ha:77-119
@@ -7,7 +7,7 @@
 // to c3 (abs/local unimplemented); only the relative rows run here.
 //
 // Parallel `[N]str` row arrays (rather than `[N]struct{...}`) sidestep
-// the cstage cgen chained `arr[i].field` store gap (getopttest.ww:6).
+// the cstage cgen chained `arr[i].field` store gap (getopt_test.ww:6).
 // push is stateful, so each row is applied in sequence to one buffer;
 // the table holds {segment, expected-string} pairs (stack.ha:36-42 is
 // the normalization spec these rows encode).