Exclude IEnumerable.GetEnumerator() from coverage

Currently some IEnumerable.GetEnumerator() implementations are being tested and/or counted for code test coverage.

This is pointless. These methods don't need to be tested and they shouldn't impact coverage stats.

Go through and refactor these cases following the example below.

/// <inheritdoc cref="IEnumerable.GetEnumerator"/>
[ExcludeFromCodeCoverage]
IEnumerator IEnumerable.GetEnumerator() => this.GetEnumerator();