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.

test-user-mode-in-tree

@@ -1,18 +1,8 @@
 #!/usr/bin/env python3
 
-import imp
-import os
-import subprocess
+import common
 
-git_root = subprocess.check_output([
-    'git',
-    'rev-parse',
-    '--show-toplevel',
-]).decode().rstrip()
-test_user_mode = imp.load_source(
-    'test_user_mode',
-    os.path.join(git_root, 'test-user-mode')
-)
+test_user_mode = common.import_path_relative_root('test-user-mode')
 
 class Main(test_user_mode.Main):
     def __init__(self):