[biotime] Add option for testing performance of writing

Previously biotime would only read from the block device.

Bug: ZX-2466
Test: /boot/test/sys/biotime-test
Change-Id: I7393aec0f0172045c0254ee3adb452b475235c59

[biotime] Add an automated test for biotime

The test just runs biotime on a ramdisk and checks that it returns a
success status.

There was no automated test for biotime in the Zircon build before.

Bug: ZX-2466
Test: /boot/test/sys/biotime-test

Change-Id: I28887858e40af8a3909d4f8f77bda309ccdf2129