Fix exception when targeting .NET Framework with .NET 7 installed

[YuhanKun]

Related to:
#1176
#1014

[dnfadmin]

All CLA requirements met.

[manfred-brands]

LGTM, Only 6 months after I added the line for 6.

[CharliePoole]

I see no problem with this so I'm merging. The Linux and MacOS failures in azure are a separate problem.

Note that this change does not enable you to run tests targeting .NET 7.0. As with net 6.0, that requires more work.

@YuhanKun Thanks for your contribution.

@manfred-brands Welcome! I see you accepted my invitation. It's a bit annoying that GitHub doesn't notify me when someone accepts. Please contact me by email so we can talk about your participation (charliepoole AT gmail DOT com)

[CharliePoole]

Sorry... I indicated I was merging this four days ago but I appear not to have done so. Merging now!

[ankushjain358]

.Net 7 is released today and nunit is working fine. It's tested using NUnit3TestAdapter v4.3.0

[CharliePoole]

@ankushjain358 True but not relevant here. This is about testing with the console runner, not the test adapter. :-)

However, console runner version 3.16.0-dev00073 does support testing under today's release of .NET 7.0. The full release of 3.16 will be out next week.