Collection types nested inside of Input<Union<...>> types need to abide
by the usual rules for collection types nested inside of input types.
These changes replace the use of the generic SimplifyInputUnion with a
.NET-specific simplifyInputUnion that does not remove Input types
inside of a union if those Input types wrap collection types. Retaining
these Input types allows the usual logic for handling
Input<Collection<...> types in typeString to kick in.
Fixes#7569.
The inputs and expected outputs for the tests are encoded using a
schema. Each property present in the schema forms a testcase; the
expected outputs for each language are stored in each property's
`Language` field with the language name "test". Expected outputs can be
regenerated using `PULUMI_ACCEPT`.
