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/memio/memiotest.ww

@@ -524,17 +524,3 @@ fn putstr(s: str, into: []u8, off: i32) i32 = {
 	let c: (void | io.error) = io.close(s);
 	match (c) { case void => {}; case let e: io.error => abort(); };
 };
-
-export fn main() i32 = {
-	fixedread();
-	fixedwritecases();
-	dynamicgrowcases();
-	dynamicreset();
-	borrowedreadcases();
-	stringview();
-	dynamicfromseed();
-	seekcases();
-	emptyseek();
-	dynamicseek();
-	return 0;
-};