Updates to test infrastructure:
- TestStatus carries raw PlaceHolder objects

- FileShader placeholder now takes an optional assembly_substr.
  If present, that's a substring expected to be in the dissasembled
  output of an module generated from that placeholder's input.

- Add test mixin classes to check for substring in assembly
  output per shader input file.

- Pass the path to the spirv-dis to the tests, and store
  it in the test manager class.