Enable ProcessTest.THREADSAFE_Http2 on Windows.

TEST(ProcessTest, THREADSAFE_Http2) enabled in https://github.com/apache/mesos/commit/82f3ab1a0da9e85e5354d8feeb88bf4a8f1ec6f8

Appears to be this line:
https://github.com/apache/mesos/blob/master/3rdparty/libprocess/src/tests/process_tests.cpp#L1380

"Does not actually create a socket connection since the libprocess sender is equal to the local OS process." - Joseph

Unfortunately causing CI tests to fail.