test: parallelize test/run via xargs -P; 180s per-tool timeout

test/run fans test/wcc/*.c across $(nproc) workers via xargs -0 -n2 -P
$JOBS; each worker writes <prefix>.status and an optional <prefix>.fail
sentinel into a mktemp results dir. The driver collects in glob order so
output stays deterministic across runs.

Wraps every ww/w6c/w6a/w6l/wwdump invocation in 990-995 with
`timeout 180`, bounding orphan compilers under 8-way contention. The
direct runwait(exe) calls for fresh-built test binaries get the same
treatment via a "timeout 180 %s" snprintf hop. Motivation: pid 2101 was
a w6c_ww that ran 42h on /tmp/wcas_asm_1326_15.ww until we killed it
during this session; timeout makes that impossible.

993's /tmp/ww_d_hello.ww is now pid-keyed (snprintf %d getpid); without
it concurrent runs (or future shards of 993 itself) would race on the
staging file.

Wall: 9m59s → 4m31s on 8 cores. Same 132/132 status.

test/wcc/995_self_rebuild.c

@@ -79,12 +79,12 @@ rebuild_one(const char *bin, const char *cwd, const char *tool,
 
 	if (inc_local && inc_local[0]) {
 		snprintf(cmd, sizeof cmd,
-		    "cd %s && %s/ww_ww build -I %s/%s -I %s/lib/ww -I %s/lib/ww/lex -I %s/lib/ww/parse -I %s/selfhost/cmd/wcc "
+		    "cd %s && timeout 180 %s/ww_ww build -I %s/%s -I %s/lib/ww -I %s/lib/ww/lex -I %s/lib/ww/parse -I %s/selfhost/cmd/wcc "
 		    "%s/%s >/dev/null 2>&1",
 		    workdir, bin, cwd, inc_local, cwd, cwd, cwd, cwd, cwd, src_rel);
 	} else {
 		snprintf(cmd, sizeof cmd,
-		    "cd %s && %s/ww_ww build -I %s/lib/ww -I %s/lib/ww/lex -I %s/lib/ww/parse -I %s/selfhost/cmd/wcc "
+		    "cd %s && timeout 180 %s/ww_ww build -I %s/lib/ww -I %s/lib/ww/lex -I %s/lib/ww/parse -I %s/selfhost/cmd/wcc "
 		    "%s/%s >/dev/null 2>&1",
 		    workdir, bin, cwd, cwd, cwd, cwd, cwd, src_rel);
 	}