Improve System Test performance with stubbing dotnet new

[gasparnagy]

🤔 What's changed?

The TestProjectGenerator that is used by System Tests (and currently Specs tests as well) has been improved:

• the dotnet new sln and the dotnet new classlib commands have been replaced by a "stub" version that calls the original command first, but caches the result in the temp folder and for the next time it only copies the files
• the dotnet sln add command has been replaced by a stub implementation that does the same by editing the .sln file directly

⚡️ What's your motivation?

The main motivation is performance as these three basic command take significant time in local and CI execution.

With the changes I have achieved a good performance improvement locally:

• execution time of all system tests were 213 s (3:33) before
• with the changes and empty cache: 166 s (2:46), 22% improvement
• with the changes and populated cache: 158 s (2:38), 26% improvement

I also hope that this fixes #249 too.

🏷️ What kind of change is this?

• 🏦 Refactoring/debt/DX (improvement to code design, tooling, etc. without changing behaviour)

♻️ Anything particular you want feedback on?

It can happen that this changed the behavior of #109, so that needs to be retested.

📋 Checklist:

• I've changed the behaviour of the code

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[obligaron]

I added a fix for the Linux build (a replacement for the dictionary separator / was missing).
I also checked that #109 still worked. It was necessary to bind the cache to the sdk version. This will also help if different sdk versions produce different output in the future (e.g. different .sln format).

[gasparnagy]

@obligaron super, thx! Could you please make a review of the rest of the PR?

[gasparnagy]

Update: Ignore this, I added this command already.
@obligaron Also folks here mentioned that we could add a --no-restore option for the new classlib for better performance. Can you try that out?

[obligaron]

lgtm 🙂