Test a matrix of MinGW versions

[dagood]

Occasionally upstream Go has issues with specific MinGW versions and specific versions of Windows. We should be able to add tests for multiple versions to our CI to catch some of them.

I wrote a tool to install various MinGW versions that could be used for this in CI, and also locally when developing a change that might cause a problem: microsoft/go-infra#95.

• Mingw erros with specific golang versions #1081 (comment)