[spatial svc]Add a few different encode frame tests.

[spatial svc]Add a few different encode frame tests.

1. Clean the code for encode frame tests
2. Add encode w/ and w/o alt reference frame test
3. Add encode SNR layers test
4. Add encode multiple layers but decode partial layers test

Change-Id: Ibd2c9bc02525db584a6f931a98405f2d851b3cd6