nunit: CI timeout: NUnit.Framework.Assertions.CollectionAssertTest.PerformanceTests

It’s only .NET Framework 3.5 for most CI errors, it seems.

1. Failed : NUnit.Framework.Assertions.CollectionAssertTest.PerformanceTests(System.Linq.Enumerable+<RangeIterator>d__b1) Elapsed time of 174.8091ms exceeds maximum of 100ms
2. Failed : NUnit.Framework.Assertions.CollectionAssertTest.PerformanceTests(System.Collections.Generic.List`1[System.Double]) Elapsed time of 201.1894ms exceeds maximum of 100ms

https://ci.appveyor.com/project/CharliePoole/nunit/build/3.9.0-dev-04713#L1241

Just need to bump up the timeout, I think.