[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/sbstructureddata/TestStructuredDataAPI.py

@@ -11,8 +11,6 @@ from lldbsuite.test import lldbutil
 
 
 class TestStructuredDataAPI(TestBase):
-
-    mydir = TestBase.compute_mydir(__file__)
     NO_DEBUG_INFO_TESTCASE = True
 
     def test(self):