use perl instead of sed -r to strip colors in test

`sed -r` is not present on some BSD based systems, including MacOSX
Darwin.  perl is standard pretty much everywhere, so this a more
reliable way to test.

Since this is only used for tests, any performance differences
should not matter significantly.

test/support/test_helper.sh

@@ -15,7 +15,7 @@ fi
 
 # Strip color codes from a string
 strip_colors() {
-  sed -r "s/\x1B\[([0-9]{1,2}(;[0-9]{1,2})?)?[m|K]//g"
+  perl -pe 's/\e\[[\d;]*m//g'
 }
 
 # Print space separated tab completion options