Build tests in Linux x64 leg

[NikolaMilosavljevic]

Currently, tests are building in Windows x64 leg. As this is our longest build leg, and takes almost 2 hours on its own, building tests adds about 25 minutes on top and makes our PR validations longer, close to 2.5 hours. More tests will be enabled in the future, i.e. aspnetcore, which will further increase this time.

In order to keep the longest leg under 2 hours, we should build tests in Linux x64 leg, instead.

[MichaelSimons]

As part of this, we should preserve the current Windows x64 leg building tests in CI