Speed up build and testing by using solutions

[ErikSchierboom]

See: https://github.com/dotnet/cli/issues/5099

Note: this is related to PR #199.

[ErikSchierboom]

The solution file has been added, and we can get some huge speed-ups by restoring and building a solution instead of the individual projects. A small, not-so-scientific test reduced the restore time by two minutes! I expect similar gains when building the projects. Unfortunately, there is no solution-level test support yet, but it has been logged as an issue.

[ErikSchierboom]

To simplify our build script, we could create separate solution files for the different build purposes. So one Exercises.tests.sln, one Exercises.refactoring.sln and Exercises.build.sln.

[tushartyagi]

PR #225 is using solution based builds. Build times do go down substantially when using solutions, as the builds are now done in parallel.

[ErikSchierboom]

This has been fixed in #226.