tests/tcg: fix diff-out pass to properly report failure

A side effect of piping the output to head is squash the exit status
of the diff command. Fix this by only doing the pipe if the diff
failed and then ensuring the status is non-zero.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>

tests/tcg/Makefile

@@ -45,7 +45,11 @@ run-test = $(call quiet-command, timeout $(TIMEOUT) $2,"TEST",$3)
 endif
 
 # $1 = test name, $2 = reference
-diff-out = $(call quiet-command, diff -u $1.out $2 | head -n 10,"DIFF","$1.out with $2")
+# to work around the pipe squashing the status we only pipe the result if
+# we know it failed and then force failure at the end.
+diff-out = $(call quiet-command, diff -q $1.out $2 || \
+                                 (diff -u $1.out $2 | head -n 10 && false), \
+                                 "DIFF","$1.out with $2")
 
 # $1 = test name, $2 = reason
 skip-test = @printf "  SKIPPED %s on $(TARGET_NAME) because %s\n" $1 $2