test: Change loop calibration count initialization

The kernel/context test_timer_interrupts() test has a loop calibration
phase to estimate the number of loops needed for 1 system tick.
By initializing this calibration to 1 instead of 0, we can avoid an
edge condition where the calibration phase indicates 0 loops are
required (a case that could happen when running on a slow simulator).

Signed-off-by: Peter Mitsis <peter.mitsis@intel.com>