Disable acpi_timer_test by default

This disables testing the ACPI timer by default, forcing the use of
ACPI-fast rather than ACPI-safe. The broken-ACPI-timers workaround
can be re-enabled by setting the hw.acpi.timer_test_enabled=1 tunable.

This speeds up the FreeBSD boot process by 140 ms on an EC2 c5.xlarge
instance.

This change will not be MFCed.

Assuming no problems are reported, acpi_timer_test, the associated
tunable, and the ACPI-safe timecounter should be removed in FreeBSD 15.

Relnotes: The ACPI-safe timer is disabled in favour of ACPI-fast;

		if timekeeping issues are observed, please test with
		hw.acpi.timer_test_enabled=1 in loader.conf and report
		if that fixes the problem.