Bump MSTest.TestFramework from 3.4.1 to 3.4.3 in /core/getting-started/unit-testing-vb-mstest/PrimeService.Tests

Bump MSTest.TestFramework from 3.4.1 to 3.4.3 in /core/getting-started/unit-testing-vb-mstest/PrimeService.Tests #1952