Add featureflag to enable/disable pre-filtering

[OsirisTerje]

We have issues in some cases with pre-filtering, ref #648, #649, #650.

In order to avoid this, we'll add a feature flag to the runsettings file, which will be default off. You will then have to opt-in to get the performance boost.

In short what is affected:

• Any version of NUnit below 3.11.0
• Any test assemblies using SetupFixture
• TestCaseData using SetName instead of SetArgDisplayNames.

The SetupFixture has to be fixed in the framework, See nunit/nunit#3356 , and when this is fixed we can consider moving pre-filtering option to default on.

Information on enabling it, see https://github.com/nunit/docs/wiki/Tips-And-Tricks#PreFilter

[OsirisTerje]

Pre-release here: There is now a beta version of the fix in https://www.myget.org/feed/nunit/package/nuget/NUnit3TestAdapter/3.15.1-dev-01134 . Would appreciate if you checked it.

[OsirisTerje]

3.15.1 hotfix released now.