Fix bugs in power HAL VTS test

Ensure CPU governor list is null-terminated; stop treating newline
char as a governor name; stop leaking file descriptors on failure.

Bug: 35753237
Test: Test runs and passes on marlin and bullhead
Change-Id: I8a6409a50e5daac36770dc577d0628fec8148847
Signed-off-by: Connor O'Brien <connoro@google.com>