FCS: Tests for partial type checking

[majocha]

We have no unit tests whatsoever for the set of optimizations (#10199, #10210, #10650) under the enablePartialTypeChecking flag.

This is destined to rot otherwise.

For example there was caching of parsed inputs as part of #10199, that silently stopped working.

[majocha]

There exists this test, effectively for #10199:

fsharp/tests/FSharp.Compiler.ComponentTests/FSharpChecker/CommonWorkflows.fs

Line 73 in fdba974

let ``Files depend on signature file if present`` () =