add auth_client tests (using GTest) and run them from the integration test

authserv/test/test_integration.py

pam/Makefile.am

pam/auth_client.h

pam/auth_client_test.cc

pam/configure.ac

pam/gtest/include/gtest/gtest-death-test.h

pam/gtest/include/gtest/gtest-message.h

pam/gtest/include/gtest/gtest-param-test.h

pam/gtest/include/gtest/gtest-param-test.h.pump

pam/gtest/include/gtest/gtest-printers.h

pam/gtest/include/gtest/gtest-spi.h

pam/gtest/include/gtest/gtest-test-part.h

pam/gtest/include/gtest/gtest-typed-test.h

pam/gtest/include/gtest/gtest.h

pam/gtest/include/gtest/gtest_pred_impl.h

pam/gtest/include/gtest/gtest_prod.h

pam/gtest/include/gtest/internal/gtest-death-test-internal.h

pam/gtest/include/gtest/internal/gtest-filepath.h

pam/gtest/include/gtest/internal/gtest-internal.h

pam/gtest/include/gtest/internal/gtest-linked_ptr.h