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/utf8/utf8test.ww

@@ -614,43 +614,3 @@ fn streq(a: str, b: str) bool = {
 		i += 1;
 	};
 };
-
-export fn main() i32 = {
-	runesz_ranges();
-	utf8sz_classify();
-	encode_ascii();
-	encode_two_byte();
-	encode_three_byte();
-	encode_four_byte();
-	decode_one_byte();
-	decode_two_byte();
-	decode_three_byte();
-	decode_four_byte();
-	decode_surrogate();
-	decode_overlong();
-	decode_out_of_range();
-	decode_bad_continuation();
-	decode_max_in_range();
-	decode_truncated();
-	decode_done();
-	validate_mixed_ok();
-	validate_malformed();
-	validate_empty_ok();
-	roundtrip();
-	prev_done_at_start();
-	prev_one_byte();
-	prev_two_byte();
-	prev_three_byte();
-	prev_four_byte();
-	prev_mixed_roundtrip();
-	prev_continuation_only_more();
-	prev_incomplete_invalid();
-	prev_surrogate_invalid();
-	prev_overlong_invalid();
-	prev_extracont_invalid();
-	prev_max_in_range();
-	prev_min_out_of_range();
-	remaining_slice_position();
-	strerror_cases();
-	return 0;
-};