run-tests: don't append rtendtracing logs if pipe is /dev/null

If HGCATAPULTSERVERPIPE is not set, we set it to /dev/null, so we need to make
sure the value is not /dev/null before appending logs.

This fixes some of test-run-tests.t failures.

Differential Revision: https://phab.mercurial-scm.org/D4401

tests/run-tests.py

@@ -1389,7 +1389,8 b' class TTest(Test):'
         if os.getenv('MSYSTEM'):
             script.append(b'alias pwd="pwd -W"\n')
 
-        if os.getenv('HGCATAPULTSERVERPIPE'):
+        hgcatapult = os.getenv('HGCATAPULTSERVERPIPE')
+        if hgcatapult and hgcatapult != '/dev/null':
             # Kludge: use a while loop to keep the pipe from getting
             # closed by our echo commands. The still-running file gets
             # reaped at the end of the script, which causes the while