KVM: arm64: selftests: Refactor debug-exceptions to make it amenable to new test cases

Split up the current test into a helper, but leave the debug version
checking in main(), to make it convenient to add a new debug exception
test case in a subsequent patch.

Signed-off-by: Reiji Watanabe <reijiw@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220917010600.532642-4-reijiw@google.com