php: add more unit tests to improve code coverage

src/php/ext/grpc/call.c

src/php/ext/grpc/channel.c

src/php/tests/unit_tests/CallCredentials3Test.php

src/php/tests/unit_tests/CallCredentialsTest.php

src/php/tests/unit_tests/CallTest.php

src/php/tests/unit_tests/ChannelCredentialsTest.php

src/php/tests/unit_tests/ChannelTest.php

src/php/tests/unit_tests/EndToEndTest.php

src/php/tests/unit_tests/ServerTest.php

src/php/tests/unit_tests/TimevalTest.php