[CI][run_test] Fix rerun logic for failing at exit (#155853)

Sometimes a test file reports success according to pytest, but fails afterwards, and the rerun logic doesn't handle that correctly.

The name of the last run test is saved in order to do more efficient reruns (target the last run test for a rerun without rerunning the entire file).  This usually correct, ex test fails and pytest catches it -> lastrun = the test that failed, test segfaults (pytest doesn't catch) -> lastrun is the test that segfaulted.  But sometimes pytest reports a success, but the process has non zero exit code.  The two cases I know of are hangs and double freeing at exit.  In this case, its unclear which test caused the failure, so lastrun is set to be the first test that ran in that session, so that during the next session it will start from the beginning in an attempt to replicate the error (an alternate solution would be to just fail and not rerun, which might be the better option).  But then it reruns with runsingle, which prevents lastrun from being reset (not sure why, I'm pretty sure there's no difference between resetting and not normally), so lastrun becomes the last test that ran, and its not always true that lastrun is the one that caused it. Then on the next run, it starts from the last test and the process now exits cleanly

Short term solution here: ensure the lastrun is always set to the initial value if the session succeeds.  This is correct even in the normal path because initial value shouldn't change in that case

Things that still need to be fixed:
* log says "running single test" which is not true
* no xml reports get generated here
* also no xml reports get generated on segfault
* docs for this

I think I have a PR that fixes the above but its old so I need to take another look

Testing:
This from when I was based on a commit that had a hang for macs, and before I added the skips in inductor array ref:
cc862d2c14

Pull Request resolved: https://github.com/pytorch/pytorch/pull/155853
Approved by: https://github.com/malfet

test/conftest.py

@@ -341,5 +341,5 @@ class StepcurrentPlugin:
         self.cache.set(self.directory, self.lastrun)
 
     def pytest_sessionfinish(self, session, exitstatus):
-        if exitstatus == 0 and not self.run_single:
+        if exitstatus == 0:
             self.cache.set(self.directory, self.initial_val)