Mono is unstable with these tests. I've seen it fail several tests on my computer and succeed the next time. It's happening in Azure Pipelines too. Right now 7 tests are failing in AzP related to a SharedData_FirstSimpleDataFormatter73 type load failure. Since we're using static constructors everywhere in MessagePack, one failure will cascade to all subsequent uses of that type.

I think several of the tests I've 'skipped' already for mono were of this same variety, where the tests themselves may not have been at fault, but perhaps were a casualty of a prior failure. I need to work on this more...

So the irony is that it's all just one bug in mono, and I think it has to do with either MessagePack's dynamic code generation or heavy use of static generic types. Either way, the tests all pass on mono, provided they run sequentially. When run in parallel, it fails very frequently on a random set of tests.

Filed mono/mono#20369 to track the concurrency bug on the mono side. In the meantime, our -parallel none switch is keeping the tests passing reliably in my testing.