Get rid of imp, started giving deprecation warning every time in Python 3.7 in Ubuntu 19.04.

Please python stop torturing me with refactors.

Make ./run -u blow up if executable not found, otherwise I go crazy.

Get ./test-gdb back to life after the ./run relative path refactor, forgot to test this.

run-gdb

@@ -1,6 +1,5 @@
 #!/usr/bin/env python3
 
-import imp
 import os
 import signal
 import subprocess
@@ -34,7 +33,7 @@ class GdbTestcase:
         self.child.setecho(False)
         self.child.waitnoecho()
         self.child.expect(self.prompt)
-        test = imp.load_source('test', test_script_path)
+        test = common.import_path(test_script_path)
         exception = None
         try:
             test.test(self)