tests: fix ordering of PYTHONPATH (#1210)

* tests: fix ordering of PYTHONPATH
If you run the sitecustomize test cases on their own they fail because
they import the ddtrace/bootstrap/sitecustomize.py module and not
the tests/commands/bootstrap/sitecustomize.py module.

This is because this helper code would put the test bootstrap module
on the end of the PYTHONPATH, which means it gets looked at last.

I have no idea how this seems to work ok when you run the entire
test suite together, must be an odd dependency on a previous test
case?

* fix flake8

tests/commands/test_runner.py

@@ -22,10 +22,10 @@ def inject_sitecustomize(path):
     env = os.environ.copy()
     sitecustomize = os.path.join(root_folder, '..', path)
 
-    # Add `boostrap` module so that `sitecustomize.py` is at the bottom
-    # of the PYTHONPATH
-    python_path = list(sys.path) + [sitecustomize]
-    env['PYTHONPATH'] = ':'.join(python_path)[1:]
+    # Add `bootstrap` directory to the beginning of PYTHONTPATH so we know
+    # if `import sitecustomize` is run that it'll be the one we specify
+    python_path = [sitecustomize] + list(sys.path)
+    env['PYTHONPATH'] = ':'.join(python_path)
     return env