Implement test config auto-gen for sh_test.

Supports the following properties:
- Require_root
- Test_config_template
- Auto_gen_config

Test config template is in a separate CL.

Bug: 148805488
Bug: 151761373

Test: make
Test: unit tests
Test: verified with bc_test & toybox-tests
Change-Id: I63d60a153a6abda4ce30b74d3eb915fbf6389cb8