arm: timer: Move the different tests into their own functions

At the moment, the timer test is one big function that checks different
aspects of the timer implementation, and it's not immediately obvious if a
check depends on a state from a previous test, making understanding the
code more difficult than necessary.

Move the checks into logically distinct functions and leave the timer in a
known state before proceeding to the next test to make the code easier to
understand, maintain and extend.

The timer interrupt is now enabled at the GIC level in test_init() to break
the dependency that the tests have on the timer pending test running first.

There should be no functional change as a result of this patch.

Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
Message-Id: <20211207154641.87740-3-alexandru.elisei@arm.com>
Signed-off-by: Andrew Jones <drjones@redhat.com>