Add test for MSBuild Server

[Forgind]

Progress on #7377

Context

Adds a test for MSBuild server. This covers

• ensuring the server starts when requested,
• ensuring the requesting node dies thereafter,
• ensuring the server persists for future builds,
• ensuring that the server node can be killed even in the middle of a build without preventing the next build from succeeding,
• ensuring that that next build also uses the MSBuild server node but it has a different process ID from the previous MSBuild server node.

Changes Made

Added one test and modified RunnerUtilities.ExecMSBuild to support it.

Testing

Ran the test, and it passed. Observed intermediate stages to ensure they were as expected.

Notes

Other tests to add:
Verify legacy behavior
Verify proper mixed behavior (legacy build after MSBuild server started)
Check behavior when starting a new build while the previous build is still executing.

[rokonec]

I think 3s it way too long. I would start with 1s, put comment there "if flaky on 'Server already running' enlarge this time".
The things is that we shall, strategically, try to have fast unit tests.

[danmoseley]

I'm likely not understanding the test, but process IDs get reused, and the algorithm used is not stable, so if a server process exits the next one can potentially get the same id?

[AR-May]

The tests should check the logic of msbuild server. That is:

• If the special flag is on, we use msbuild server for task execution. Msbuild server process is a long living one. It lives the same time as the msbuild nodes (15 min).
• If server is occupied, there should be a fallback to the usual, non-server, build.

The task itself writes down the id of the process that is used for the build, and here we write down the process id of the msbuild process that was created for building a project. If msbuild server was used, these ids would be different (first one would be the id of msbuild server and the second one would be the id of msbuild client). If not used, they would be the same. Thus, we check how the build was executed, in or out the server.

[danmoseley]

Ah, so long as the first process has not exited, then yes of course they have different PID's. Thanks. Otherwise of course you have to check process start time.