A more robust pexpect-based test case for testing against the firing of stop hooks.

Change one test sequence to detect the '** End Stop Hooks **' marker emitted by the
stop hooks mechanism and check for whether the 'expr ptr' stop-hook has been run.

Also, change the TestBase.tearDown() to wait for 2 seocnds before forcefully kill
the pexpect-spawned child lldb process.

llvm-svn: 130767