testsuite: Fix dependence on grep behavior in T8129

grep was considering the output to be binary on my machine, resulting in a
spurious test failure.

testsuite/tests/indexed-types/should_fail/Makefile

@@ -14,5 +14,5 @@ T8227:
 # an asertion failure with -ddump-tc-trace
 T8129:
 	-'$(TEST_HC)' $(TEST_HC_OPTS) -c -ddump-tc-trace T8129.hs 2> T8129.trace > T8129.trace
-	grep deduce T8129.trace
+	grep -a deduce T8129.trace