Improve project extension test diagnostics

[rainersigwald]

Project-extension import tests were relying on a shared temp directory and weak assertions, so failures could be opaque and environment-sensitive. This change isolates each test case and adds explicit diagnostics so the DoesNotImportProjectIfNotExist path is easier to reason about and reproduce.

Changes Made

• moved the shared import fixture to isolated TestEnvironment/TransientTestFolder state per test
• added ITestOutputHelper logging for the evaluated MSBuildProjectExtensionsPath
• tightened assertions with explicit Shouldly checks and ShouldHaveSingleItem() expectations
• aligned the props/targets subclasses with the new fixture pattern

This should improve a flaky failure that hit #14280.