Add -g flag to run only tests matching a pattern (grep)

(Actually, it's only matching strings for me, not patterns, but according to
Mocha it's supposed to compile the patterns using RegExp.)

test/content/runtests.js

@@ -79,7 +79,8 @@ function Reporter(runner) {
 mocha.setup({
 	ui: "bdd",
 	reporter: Reporter,
-	timeout: 5000
+	timeout: 5000,
+	grep: ZoteroUnit.grep
 });
 
 // Enable Bluebird generator support in Mocha