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/errors/errnotest.ww

@@ -104,10 +104,3 @@ fn errtag(e: errors.error) int = {
 	assert(!(!streq(os.strerror(os.EINVAL), "Invalid argument")));
 	assert(!(!streq(os.strerror(5), "Unknown error")));
 };
-
-export fn main() i32 = {
-	mapping();
-	opaquetail();
-	strerrortext();
-	return 0;
-};