tests: kernel/cache: skip i-cache range tests if Xtensa MMU

With MMU enabled on Xtensa, user_buffer is not marked as
executable. Invalidating the i-cache by region will cause
an instruction fetch prohibited exception. So skip all
i-cache tests, instead of just the range ones to avoid
confusions of only running the test partially.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>