tests/kernel/common: Remove needless 1cpu limitation from test_clock_uptime

It's not at all clear to me why this was set to 1cpu, it's a single
thread doing sequential things.  (I tripped over it because the 1cpu
happened to tickle an unrelated arm64 bug with interrupt state.  But
we might as well fix it here.)

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>