bionic tests: migrate test for mutex type from system/extras

Migrate the test about pthread mutex type in file
    system/extras/tests/bionic/libc/bionic/test_mutex.c
to the new place
    bionic/tests/pthread_test.cpp
in the gtest format.

Change-Id: I6aab10170ccad5b9a4892d52dba2403876c86659
Signed-off-by: Yongqin Liu <yongqin.liu@linaro.org>