Fixed bad test expectations in String Tests

[SandorDobos]

LastIndex* methods are searching backwards from their startIndex param!

Description

Test expectations were bad.

Motivation and Context

• Fixes UnitTestStringTests.LastIndexOf tests are failing Home#710

How Has This Been Tested?

Test commands were executed on .NET fw 4.7,2 which revealed the bad expectations.
After fixing nf corelib tests they become green.

Screenshots

Types of changes

• Improvement (non-breaking change that improves a feature, code or algorithm)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Config and build (change in the configuration and build system, has no impact on code or features)
• Dependencies (update dependencies and changes associated, has no impact on code or features)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[Ellerbach]

Thanks for fixing the test cases! Missed that one!