tests: spi_loopback: Specify expected CS on helper func

The helper function should have the flexibility to have different
expected CS counts. Namely, the test cases where the transfer is
essentially empty should not need to assert CS and many platforms are
failing on CS testing currently due to expecting CS assert and deassert
despite it being a 0 clock/bit transfer being specified.

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>