[lldb][tests] Automatically call compute_mydir (NFC)

Eliminate boilerplate of having each test manually assign to `mydir` by calling
`compute_mydir` in lldbtest.py.

Differential Revision: https://reviews.llvm.org/D128077

lldb/test/API/python_api/thread/TestThreadAPI.py

@@ -14,8 +14,6 @@ from lldbsuite.test.lldbutil import get_stopped_thread, get_caller_symbol
 
 class ThreadAPITestCase(TestBase):
 
-    mydir = TestBase.compute_mydir(__file__)
-
     def test_get_process(self):
         """Test Python SBThread.GetProcess() API."""
         self.build()