Add `UseSpecFlowNUnitFeatures` method to `AtataContextBuilder`

[YevgeniyShunevych]

Enables all Atata features for SpecFlow+NUnit. Executes the following methods:

• UseNUnitTestName()
• UseNUnitTestSuiteName()
• UseNUnitTestSuiteType()
• UseNUnitAssertionExceptionType()
• UseNUnitAggregateAssertionStrategy()
• UseNUnitWarningReportStrategy()
• UseNUnitAssertionFailureReportStrategy()
• EventSubscriptions.LogNUnitError()
• EventSubscriptions.TakeScreenshotOnNUnitError()
• EventSubscriptions.TakePageSnapshotOnNUnitError()

The method is basically analogous to UseAllNUnitFeatures except it lacks EventSubscriptions.LogNUnitError() and EventSubscriptions.AddArtifactsToNUnitTestContext() calls.