When code coverage is not being collected, just issue a single 'go test'
command, as is already done. Go will internally parallize execution.

When code coverage is being collected, it is necessary to issue separate
'go test' commands for each package, since Go does not support
collecting coverage across packages. Using xargs -P will parallelize
these invocations, however, speeding up test execution.
The number of simultaneous processes to use can be specified with
KUBE_COVERPROCS.

Update Travis config to pass along the number of CPUs to use for running
tests.