pngvalid tests: add --strict, fix test names

Signed-off-by: John Bowler <jbowler@acm.org>
diff --git a/tests/pngvalid-gamma-threshold b/tests/pngvalid-gamma-threshold
index 2cfca91..3fe4edf 100755
--- a/tests/pngvalid-gamma-threshold
+++ b/tests/pngvalid-gamma-threshold
@@ -1,2 +1,2 @@
 #!/bin/sh
-exec ./pngvalid --gamma-threshold
+exec ./pngvalid --strict --gamma-threshold