tests: drivers: regulator: clean up testcase.yaml for voltage test

Clean up testcase definition for voltage test, by moving the
LPCXpresso55s36 board overlay file into the boards/ folder of the test,
as well as adding a fixture to the testcase so that it will only be
executed on properly configured boards.

Fixes #62884

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>