`test_platform_prober` assumes the `x11` platform is built

[Saviq]

When x11 is excluded from MIR_PLATFORM, the tests fail to build:

/usr/bin/ld: tests/unit-tests/CMakeFiles/mir_umock_unit_tests.dir/graphics/test_platform_prober.cpp.o: in function `testing::NiceMock<mir::test::doubles::MockX11>::~NiceMock()':
/usr/include/gmock/gmock-nice-strict.h:149:(.text._ZN7testing8NiceMockIN3mir4test7doubles7MockX11EED2Ev[_ZN7testing8NiceMockIN3mir4test7doubles7MockX11EED5Ev]+0x18): undefined reference to `mir::test::doubles::MockX11::~MockX11()'
/usr/bin/ld: tests/unit-tests/CMakeFiles/mir_umock_unit_tests.dir/graphics/test_platform_prober.cpp.o: in function `testing::NiceMock<mir::test::doubles::MockX11>::NiceMock()':
/usr/include/gmock/gmock-nice-strict.h:158:(.text._ZN7testing8NiceMockIN3mir4test7doubles7MockX11EEC2Ev[_ZN7testing8NiceMockIN3mir4test7doubles7MockX11EEC5Ev]+0x25): undefined reference to `mir::test::doubles::MockX11::MockX11()'