[test] Make sure PROMPT is a string

Currently tests using expect_prompt are failing on the Python 3 bot with
an error saying "argument must be str, not bytes". I don't have a Python
3 build handy, but I suspect this might fix that.

llvm-svn: 370526

lldb/packages/Python/lldbsuite/test/lldbpexpect.py

@@ -22,7 +22,7 @@ else:
     class PExpectTest(TestBase):
 
         NO_DEBUG_INFO_TESTCASE = True
-        PROMPT = "(lldb) "
+        PROMPT = str("(lldb) ")
 
         def expect_prompt(self):
             self.child.expect_exact(self.PROMPT)