Rework how test processes are called on Windows

This changes how runSubprocess works on Windows to insert all of the
calls within a temporary bash script. This ensures that the entire
environment is available when running the processes, which doesn't
work when simply calling subprocess.check_call().

btest

@@ -252,7 +252,17 @@ def runTestCommandLine(cmdline, measure_time, **kwargs):
 def runSubprocess(*args, **kwargs):
     def child(q):
         try:
-            subprocess.check_call(*args, **kwargs)
+            if sys.platform == 'win32':
+                tmpdir = normalize_path(kwargs.get('cwd', ''))
+                tf, tf_name = tempfile.mkstemp(suffix='.sh', dir=tmpdir)
+                fcontents = f'#!/usr/bin/env bash\n{" ".join(args)}\n'
+                with os.fdopen(tf, 'wb') as f:
+                    f.write(fcontents.encode('utf-8'))
+
+                cmd = ['bash.exe', '-c', normalize_path(tf_name)]
+                subprocess.check_call(cmd, **kwargs)
+            else:
+                subprocess.check_call(*args, **kwargs)
             success = True
             rc = 0