emul: SBS Gauge emulator reset rule

Since we have state that gets mutated between tests in the SBS fuel gauge
emulator we ought to reset its state before each test.

Add a reset rule to the SBS fuel gauge emulator that resets the state
before each test. Note: This includes allowing the emulator state to be
modified by user-threads.

Signed-off-by: Aaron Massey <aaronmassey@google.com>