Fix unit tests for mainUtils

Commit 4ab87e24f61b9aff26738cdb154673e03cd772a8 replaced too many usages
of "with self.lock as l" with "with self.lock"

This commit corrects the missing "with self.lock as l" problems to
ensure the unit tests pass
Authored-by: NTyler Ramer <tramer@vmware.com>

gpMgmt/bin/gppylib/test/unit/test_unit_mainUtils.py

@@ -24,7 +24,7 @@ class MainUtilsTestCase(GpTestCase):
         self.assertEquals(False, os.path.exists(self.lockfile))
 
     def test_lock_owned_by_parent(self):
-        with self.lock:
+        with self.lock as l:
             self.assertEquals(l.read_pid(), self.ppid)
 
 
@@ -34,7 +34,7 @@ class MainUtilsTestCase(GpTestCase):
                 self.lock.acquire()
 
     def test_child_can_read_lock_owner(self):
-        with self.lock:
+        with self.lock as l:
             pid = os.fork()
             if pid == 0:
                 self.assertEquals(l.read_pid(), self.ppid)