[TestMTCSimple] Disable the test if you don't have libMTC

If you are running on macOS and have the CommandLineTools installed of
Xcode, this test will fail because CommandLineTools doesn't ship with
libMainThreadChecker. Skip the test if you don't have it installed.

lldb/packages/Python/lldbsuite/test/functionalities/mtc/simple/TestMTCSimple.py

@@ -17,12 +17,16 @@ class MTCSimpleTestCase(TestBase):
     @skipUnlessDarwin
     def test(self):
         self.mtc_dylib_path = findMainThreadCheckerDylib()
-        self.assertTrue(self.mtc_dylib_path != "")
+        if self.mtc_dylib_path == "":
+            self.skipTest("This test requires libMainThreadChecker.dylib")
+
         self.build()
         self.mtc_tests()
 
     @skipIf(archs=['i386'])
     def mtc_tests(self):
+        self.assertTrue(self.mtc_dylib_path != "")
+
         # Load the test
         exe = self.getBuildArtifact("a.out")
         self.expect("file " + exe, patterns=["Current executable set to .*a.out"])