Tests should be run from the solution file

[Youssef1313]

why not running tests directly from the main .sln file?

Originally posted by @mizrael in #530 (comment)

[Youssef1313]

@mizrael Your point is totally valid. CI shouldn't list the test projects and run on them one by one. A couple of reasons:

1. When a new test project is added, you may forget to update the CI to run it. This is not hypothetical, it already happened and currently YamlValidator.Tests are not run in CI.
2. If you (in the future) properly configured code coverage, you will want a single report to get the right/accurate results. Maybe two test projects are using the same production assembly and one of them is covering some part while the other doesn't. You don't want to see the results of only one of the test projects that would say some part isn't covered.

cc @Evangelink