Add a unit test that checks that the returned FSharpProjectOptions graph doesn't contain duplicates

[safesparrow]

...as a proxy for checking that every project is mapped once and cached.

Test creates a "dense" graph:

• 4 layers, 2 projects in each layer
• Projects in each layer reference both projects from the previous layer

It then calls mapManyOptions and asserts that the returned graph of FSharpProjectOptions contains no duplicates ie. objects are reused.

Test results

When I removed the caching on the branch, the result was:

[23:12:19 ERR] Main tests.When creating FCS options, caches them failed in 00:00:07.4900000.
Mapping should reuse instances of FSharpProjectOptions and only create one per project.
expected: 8
  actual: 52
...
Failed:   1
        Main tests.When creating FCS options, caches them
Errored:  0 <Expecto>

When caching is enabled, test passes:

Failed:   0
Errored:  0 <Expecto>