bionic/test: migrate sysconf tests from system/extras to bionic/tests

The old test are implemented in file:
  system/extras/tests/bionic/libc/other/test_sysconf.c
This change is to migrate them to bionic/tests with the gtest format.

and since the sysconf is defined in unistd.h, will put the test under
bionic/tests/unistd_test.cpp file as unistd.syscon test

Change-Id: Ie519147c1c86a6c4cefa8c88b18bf58bdfbffbdb
Signed-off-by: Yongqin Liu <yongqin.liu@linaro.org>