Add very minimal support for a scripting dictionary — only the getURL

AppleEvent — and add an XCTestCase that can run and verify results of
AppleScripts that target Evergreen.

EvergreenTests/ScriptingTests/scripts/testGetURL.applescript

@@ -0,0 +1,19 @@
+try
+    tell application "Evergreen"
+	    open location "http://scripting.com/rss"
+    end tell
+on error message
+    return {test_result:false, script_result:message}
+end
+
+-- open location is not expected to return a value
+-- trying to access result should trigger an error, and that indicates a successful test
+
+try
+	set getURLResult to the result
+	set testResult to false
+on error message
+	set testResult to true
+end try
+
+return {test_result:testResult}