test: port the assert/abort run-stderr net to ww; retire 957_assert_builtin_run

The unowned remainder is the four run-stderr assertions — msg content
for fail_msg/abort_msg, MANDATORY EMPTINESS for fail_nomsg/abort_bare
(rt_abort(NULL,0) writes nothing) — which the fixture protocol cannot
see (run exit only). The rows build the r957_* fixture sources in
place via the cstage driver; byte-id extends the lowering to wwstage
(the carrier's own argument). Owned legs cited, not duplicated: all
eight exit rows by the r957_assert_*/r957_abort_* fixtures, the
checker rejects by r957_assert_reject_*, byte-id by the
test-data-byteid blanket.

test/misc/abortmsg_test.ww

@@ -0,0 +1,65 @@
+package abortmsg_test;
+
+// Run-STDERR content net for the #58 assert/abort builtins, ported
+// from the retired native carrier test/wcc/957_assert_builtin_run.c.
+// The eight exit-code rows and the checker rejects are owned verbatim
+// by the r957_* fixtures, and the w6c-vs-w6c_ww byte-id by the
+// test-data-byteid blanket; the fixture protocol checks run EXIT
+// only, so the run-stderr dimension — msg content for the msg forms,
+// MANDATORY EMPTINESS for the bare forms (rt_abort(NULL, 0) writes
+// nothing) — lives here. The four abort-shaped fixture sources are
+// built IN PLACE so fixture and probe can never drift; the cstage
+// driver suffices, byte-id extends the lowering to wwstage (the
+// carrier's own argument).
+
+import os;
+import os.exec;
+import strings;
+import testenv;
+import time;
+
+fn fail(label: str, why: str) void = {
+	let m: str = strings.concat("abortmsg FAIL: ", label, " -- ", why,
+		"\n");
+	os.write(2, m.ptr, m.len: u64);
+	assert(false);
+};
+
+fn tmo() time.duration = {
+	return (240i64 * (time.second: i64)): time.duration;
+};
+
+// needle "" = run stderr must be EXACTLY empty.
+fn stderrrow(label: str, fixture: str, needle: str) void = {
+	let src: str = strings.concat(testenv.repo(), "/test/wcc/data/",
+		fixture, "/case.ww");
+	let td: str = testenv.fresh();
+	let co: testenv.commandout;
+	let bav: []str = [testenv.driver("ww"), "build", "-o", "bin", src];
+	testenv.runcommand(td, td, "build", bav, tmo(), &co);
+	if (co.termination != exec.termination.EXIT || co.code != 0) {
+		fail(label, "cstage build failed");
+	};
+	let rav: []str = [strings.concat(td, "/bin")];
+	testenv.runcommand(td, td, "run", rav, tmo(), &co);
+	if (co.termination != exec.termination.EXIT || co.code != 1) {
+		fail(label, "run-exit != 1");
+	};
+	if (needle.len != 0) {
+		if (!testenv.has(co.stderr, needle)) {
+			fail(label, strings.concat("run stderr missing \"",
+				needle, "\""));
+		};
+	} else if (co.stderr.len != 0) {
+		fail(label, strings.concat("run stderr not empty ",
+			"(rt_abort no-msg must be (NULL, 0))"));
+	};
+	testenv.clean(td);
+};
+
+@test fn abortstderr() void = {
+	stderrrow("fail_nomsg", "r957_assert_fail_nomsg", "");
+	stderrrow("fail_msg", "r957_assert_fail_msg", "assert fired");
+	stderrrow("abort_msg", "r957_abort_msg", "boom");
+	stderrrow("abort_bare", "r957_abort_bare", "");
+};