Hide acpi_timer_test behind a tunable

When hw.acpi.timer_test_enabled is set to 0, this makes acpi_timer_test
return 1 without actually testing the ACPI timer; this results in the
ACPI-fast timecounter always being used rather than potentially using
ACPI-safe.

The ACPI timer testing was introduced in 2002 as a workaround for
errata in Pentium II and Pentium III chipsets, and is unlikely to be
needed in 2021.

While I'm here, add TSENTER/TSEXIT to make it easier to see the time
spent on the test (if it is enabled).

Reviewed by: allanjude, imp
MFC After: 1 week