Update tests, add root path for tests

When writing PSP tests, host0:/ points to where usbhostfs_pc was run from, e.g. from pspautotests/.  However, in PPSSPP it's always pointed to the test's directory.

This adds an option to override that behavior, and uses it by default when running tests.  This way, there's one less thing that doesn't match.

I'm not sure about the host0: = cwd behavior, so I left it alone behind a `headLess` check.

-[Unknown]