Simplify combined use of CodeActionEquivalenceKey and CodeActionIndex

[sharwell]

For a code action test with (potentially) multiple passes that specifies both CodeActionEquivalenceKey and CodeActionIndex:

• For the first pass
  • Verify that CodeActionIndex is present, and verify the code action equivalence key at that position matches CodeActionEquivalenceKey (matches current behavior)
• For subsequent passes
  • If no code action matches CodeActionEquivalenceKey, silently stop applying code fixes (differs from current behavior)
  • Otherwise, verify that the code action matching CodeActionEquivalenceKey is located at index CodeActionIndex

This change will remove the need to specify FixOne in cases where a specific code action appears once but a different code action appears in the next pass. This occurred several times in dotnet/roslyn#54310 (comment).