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/testGenericScript.applescript

@@ -0,0 +1,13 @@
+
+-- Evergreen scripting unit tests expect a dictionary to be returned from a script
+-- containing either 
+--      {test_result:true}  
+-- to indicate success or 
+--      {test_result:false}
+-- to indicate failure
+-- Data can be passed back to unit test code by including a script_result field
+-- for example this script returns "Geoducks!" as the result
+-- this can be used as part of XCTest verification
+-- see the testGenericScript() function in the ScriptingTests XCTestCase
+
+return {test_result:true, script_result:"Geoducks!"}