one more "make check" race in src/lib/log/tests

#1642 (closed) fixed some races where C++ tests, shell tests and python tests would be allowed to run concurrently. The src/lib/log/tests library slipped my attention then. You can tell by the fact that it still has a check-local in Makefile.am which should be removed and the files under check-local should be passed to TESTS so that make check knows what to run sequentially.

Here are the tests interleaved:

[ RUN      ] FormatterTest.mismatchedPlaceholders
[ RUN      ] buffer-logger.buffer-excluding-process()-call
[       OK ] buffer-logger.buffer-excluding-process()-call
[       OK ] FormatterTest.mismatchedPlaceholders (172 ms)

Also, found this warning, which seems to not be strictly related, but would be nice to be fixed:

[WARNING] /build/gtest/src/googletest-release-1.11.0/googletest/src/gtest-death-test.cc:1121:: Death tests use fork(), which is unsafe particularly in a threaded context. For this test, Google Test detected 5 threads. See https://github.com/google/googletest/blob/master/docs/advanced.md#death-tests-and-threads for more explanation and suggested solutions, especially if this is the last message you see before your test times out.

This affects distcheck which is run with multiple jobs in Jenkins, in the interest of timely results.