Bug 1654479 - Clear up processes after shutting them down in XPCShellTests. r=gbrown

When using unittest rather than mozunit, something seems to be holding
onto the XPCShellTests, which keeps the subprocess objects around even
after we shut them down, and in turn, that keeps the pipe file
descriptors associated with the subprocesses around, effectively leaking
them for the duration of the entire selftest, which in some cases could
exhaust the system resources allocated to the selftest process.

Differential Revision: https://phabricator.services.mozilla.com/D84471

testing/xpcshell/runxpcshelltests.py

@@ -1278,6 +1278,7 @@ class XPCShellTests(object):
                     self.log.info(msg)
             dumpOutput(proc.stdout, "stdout")
             dumpOutput(proc.stderr, "stderr")
+        self.nodeProc = {}
 
     def startHttp3Server(self):
         """
@@ -1356,6 +1357,7 @@ class XPCShellTests(object):
                     self.log.info(msg)
             dumpOutput(proc.stdout, "stdout")
             dumpOutput(proc.stderr, "stderr")
+        self.http3ServerProc = {}
 
     def buildXpcsRunArgs(self):
         """