tests: kernel/pipe_api: move local k_pipe var to global

When CONFIG_KERNEL_COHERENCE is enabled, pend_locked() asserts
when wait_q is not in coherent memory. The k_pipes used in
the pipe_api tests are declared locally within the test. They
are located within the thread stacks and those wait_q inside
k_pipe struct are thus not considered in coherent memory.
To make them work, replace the local ones with a global
k_pipe object. Since each test initializes the pipe object
locally, the tests are not functionally changed.

Fixes #84235

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