Changes tests to use global g_ScriptRuntime instead of creating a new JSRuntime for each test.

The tests crashed on my Debian systems but not on my Ubuntu system.
The crash happened in line 142 of SpiderMonkey's ThreadLocal.h.

I know that I had to use a workarounnd for contexts to avoid destroying
the context that was created first. I also had in mind that in newer
versions a JS_Init function got introduced which presumably solves this
kind of issues.
Based on this experience I assumed runtimes could have a similar problem
and this patch indeed fixes the issues.
Unfortunately the correct usage of JSRuntimes in that regard is not
documented.
There's only a rater misterious comment in JSAPI.h which hasn't been
cleared up so far and is most likely completely outdated
(https://bugzilla.mozilla.org/show_bug.cgi?id=992641).

This was SVN commit r14995.

source/test_setup.cpp

@@ -81,12 +81,14 @@ class MiscSetup : public CxxTest::GlobalFixture
 		ThreadUtil::SetMainThread();
 
 		g_Profiler2.Initialise();
+		g_ScriptRuntime = ScriptInterface::CreateRuntime();
 
 		return true;
 	}
 
 	virtual bool tearDownWorld()
 	{
+		g_ScriptRuntime.reset();
 		g_Profiler2.Shutdown();
 
 		return true;