KVM: selftests: Tell the compiler that code after TEST_FAIL() is unreachable

Add __builtin_unreachable() to TEST_FAIL() so that the compiler knows
that any code after a TEST_FAIL() is unreachable.

Signed-off-by: David Matlack <dmatlack@google.com>
Message-Id: <20220929181207.2281449-2-dmatlack@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>