Tests targeting NetStandard due to readonly

The default for IL baselines includes `readonly` when emitting a method signature. This means APIs which have become `readonly` will display differently depending on what references the code was compiled against. For example `Nullable.GetValueOrDefault`. In modern .NET Core TFMs it will appear as `readonly` while in others it will not. This makes it hard to have tests that run on both .NET Framework and .NET Core as the IL representations change.

These are tests that target `netstandard2.0` because of this issue.

Types:

- [ ] NullConditionalAssignmentTests
- [ ] UserDefinedConversionTests

Methods:

- [ ] CollectionExpressionTests.IOperation_01
- [ ] TargetTypedSwitch_AnyTypedSwitchWithoutTargetType

Locate the listed NullConditionalAssignmentTests, UserDefinedConversionTests, CollectionExpressionTests.IOperation_01, and TargetTypedSwitch_AnyTypedSwitchWithoutTargetType tests, then inspect why each targets netstandard2.0. Compare their IL baselines across target frameworks; completion should establish the appropriate target and consistent readonly representation for every listed test.

Written by the indexing model from the issue text.