Fix test skip test on Mac

The bash regex equality operator on Mac does not match an empty string to
all input strings as it does for Linux. This was causing all of our tests
to fail to run on Mac.

Added an explicit check for the empty case.

build/scripts/tests.sh

@@ -45,8 +45,8 @@ do
     runtimeconfig_json="${file_name%.*}".runtimeconfig.json
 
     # If the user specifies a test on the command line, only run that one
-    # "${2:-}" => take second arg, empty string if unset (regex always matches if empty)
-    if [[ ! "${file_name}" =~ "${2:-}" ]]
+    # "${2:-}" => take second arg, empty string if unset
+    if [[ ("${2:-}" != "") && (! "${file_name}" =~ "${2:-}") ]]
     then
         echo "Skipping ${file_name}"
         continue