test: use `wait-on-file` in `test-racy-mutations.t`

The official utility scale its timeout with the run-tests.py one. So lets use
it.

Differential Revision: https://phab.mercurial-scm.org/D12382

tests/test-racy-mutations.t

@@ -14,14 +14,7 b" while we're running"
   >     f="\${WAITLOCK_FILE}"
   >     start=\`date +%s\`
   >     timeout=5
-  >     while [ \\( ! -f \$f \\) -a \\( ! -L \$f \\) ]; do
-  >         now=\`date +%s\`
-  >         if [ "\`expr \$now - \$start\`" -gt \$timeout ]; then
-  >             echo "timeout: \$f was not created in \$timeout seconds (it is now \$(date +%s))"
-  >             exit 1
-  >         fi
-  >         sleep 0.1
-  >     done
+  >     $RUNTESTDIR/testlib/wait-on-file "\$timeout" "\$f"
   >     if [ \$# -gt 1 ]; then
   >         cat "\$@"
   >     fi