oeqa/concurrencytest: Ensure subunit streams are flushed at exit

Without this, error output such as that in the teardown can be lost
and processes may recieve signals they're not expecting causing other
strange errors.

(From OE-Core rev: 1e3f44737a15feb3128ba7fc0dbe896dd8782e07)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/lib/oeqa/core/utils/concurrencytest.py

@@ -225,7 +225,9 @@ def fork_for_tests(concurrency_num, suite):
                 finally:
                     if newbuilddir:
                         removebuilddir(newbuilddir)
+                    stream.flush()
                     os._exit(1)
+            stream.flush()
             os._exit(0)
         else:
             os.close(c2pwrite)