ww: lib tests run via -T test mode; bare mains retired (closes @test conversion)

'ww test' gains the istest build path (-T injection in build_one/
buildone) and do_test/dotest accept -I, mirroring do_run - both twins.
The 35 converted lib tests drop their interim bare mains (-T
synthesizes the entry from @test fns and rejects a user main); their
35 C run-drivers flip 'ww run' -> 'ww test'; 989_lib_byteid compiles
lib tests under -T (8 user-main probe fixtures stay non-T, gated on
the fixture field). Abort-on-first-failure stands until the deferred
record-and-continue harness lands with the multi-package arc.

lib/encoding/base64/base64_test.ww

@@ -188,13 +188,3 @@ fn inval_check(enc: *base64.encoding, encoded: str) void = {
 	case let e: errors.invalid => abort();
 	};
 };
-
-export fn main() i32 = {
-	rfc4648_std();
-	rfc4648_url();
-	urlsafe_distinct();
-	decode_invalid();
-	sizes();
-	roundtrip_all_bytes();
-	return 0;
-};