Reduce the test concurrency.

So many tests in Mozc have been written with an assumption that system
has plenty of idle logical CPU cores.  The factor 2x in the test
concurrency has been probably too aggressive and caused many flaky test
failures.

No behavior change is intended in production binaries.

BUG=
TEST=unittest
REF_BUG=
REF_CL=106273055

src/build_mozc.py

@@ -571,7 +571,7 @@ def ParseRunTestsOptions(args=None, values=None):
     parser.add_option('--jobs', '-j', dest='jobs',
                       default=('%d' % default_build_concurrency),
                       metavar='N', help='run build jobs in parallel')
-  default_test_jobs = GetNumberOfProcessors() * 2
+  default_test_jobs = GetNumberOfProcessors()
   parser.add_option('--test_jobs', dest='test_jobs', type='int',
                     default=default_test_jobs,
                     metavar='N', help='run test jobs in parallel')