Support hardcoding manual testcases directly in `generators.yaml`

[RagnarGrootKoerkamp]

It would be nice to directly write testcases in generators.yaml, to avoid the need for stdout.py and annoying handling of newlines.

Could be somthing like this:

testcase:
  in: >
    5
    1 2 3 4 5

Then the spec changes that instead of input being required, either in or input is required

WIP spec is here: https://github.com/Kattis/problem-package-format/pull/2/files

[RagnarGrootKoerkamp]

Alternatively we could shortcut this as

testcase.in: >
  5
  1 2 3 4 5

More generally we could say that everything with a recognised extension means hardcoded files, and everything without extension invokes a generator or manual testcase

[RagnarGrootKoerkamp]

Moved to #272