Travis: use --concurrency=multiprocessing only on build tests (#6872)

On a local workstation, it seems that tracking multiple processes during
coverage may result in malformed coverage reports for unit tests and not
for build tests.

Given that multiple processes make a difference in coverage mainly for
build tests, try to disable the tracking for unit tests to see if we get
more stable coverage results.

.coveragerc

@@ -2,7 +2,6 @@
 # .coveragerc to control coverage.py
 [run]
 parallel = True
-concurrency = multiprocessing
 branch = True
 source = lib
 omit =