Travis: Log stem's TRACE logs to a file, and tail it on failure

Also filter out stem's SocketClosed logs, and fix some nearby comments.

Part of 30591.

.travis.yml

@@ -212,7 +212,7 @@ script:
   - if [[ "$DISTCHECK" == "" && "$TEST_STEM" == "" ]]; then make check; fi
   ## Diagnostic for bug 29437: kill stem if it hangs for 9.5 minutes
   ## Travis will kill the job after 10 minutes with no output
-  - if [[ "$TEST_STEM" != "" ]]; then make src/app/tor; timelimit -p -t 540 -s USR1 -T 30 -S ABRT python3 "$STEM_SOURCE_DIR"/run_tests.py --tor src/app/tor --integ --test control.controller --test control.base_controller --test process --log TRACE | grep -v SocketClosed; fi
+  - if [[ "$TEST_STEM" != "" ]]; then make src/app/tor; timelimit -p -t 540 -s USR1 -T 30 -S ABRT python3 "$STEM_SOURCE_DIR"/run_tests.py --tor src/app/tor --integ --test control.controller --test control.base_controller --test process --log TRACE --log-file stem.log; fi
   - if [[ "$DISTCHECK" != "" && "$TEST_STEM" == "" ]]; then make distcheck DISTCHECK_CONFIGURE_FLAGS="$CONFIGURE_FLAGS"; fi
   ## If this build was one that produced coverage, upload it.
   - if [[ "$COVERAGE_OPTIONS" != "" ]]; then coveralls -b . --exclude src/test --exclude src/trunnel --gcov-options '\-p' || echo "Coverage failed"; fi
@@ -227,6 +227,7 @@ after_failure:
   - if [[ "$DISTCHECK" != "" ]]; then make show-distdir-testlog || echo "make failed"; fi
   - if [[ "$DISTCHECK" != "" ]]; then make show-distdir-core || echo "make failed"; fi
   - if [[ "$TEST_STEM" != "" ]]; then tail -1000 "$STEM_SOURCE_DIR"/test/data/tor_log || echo "tail failed"; fi
+  - if [[ "$TEST_STEM" != "" ]]; then grep -v "SocketClosed" stem.log | tail -1000 || echo "grep | tail failed"; fi
 
 before_cache:
   ## Delete all gcov files.