Limit the number of concurrent tests

[meziantou]

Every test compiles code with Roslyn, so running one test per CPU thread uses a lot of memory. On a 15 thread machine, a single test project peaks at 20 GB, and dotnet test at the solution level runs the 5 Roslyn version projects in parallel, which multiplies it.

Changes

• tests/Meziantou.Analyzer.Test/testconfig.json: limits xUnit.maxParallelThreads to 8. The file is shared by all the Roslyn versions, as their projects are in the same folder, and Microsoft.Testing.Platform.MSBuild copies it to the output folder of each project as Meziantou.Analyzer.Test.testconfig.json.
• AGENTS.md: documents the new file, and dotnet test --max-parallel-test-modules to limit how many test projects run at the same time (there is no configuration file equivalent for that one).

Measurements

Roslyn 5.9 test project (3663 tests), Release, 15 CPU threads:

Max parallel threads	Peak RSS	Duration
default (1 per CPU thread)	20.2 GB	57s
8	3.0 GB	56s
4	1.5 GB	64s

The memory grows much faster than the throughput, so 8 divides the memory by ~7 for the same duration.

Notes for the reviewer

• The value is a fixed number rather than a multiplier such as "0.5x", because the CI machines have 4 threads: a multiplier would halve their current parallelism and slow the jobs down, while 8 leaves them unchanged and only caps the bigger development machines.
• --max-parallel-test-modules bounds the number of test projects, not the memory of one project, so it does not replace this setting: a single project would still peak at 20 GB.

Verification

• dotnet test tests/Meziantou.Analyzer.Test/Meziantou.Analyzer.Test.roslyn4.8.csproj --configuration Release: 3533/3533 passed
• Roslyn 5.9 test project: 3663/3663 passed, peaking at 3.7 GB instead of 20 GB
• The generated Meziantou.Analyzer.Test.testconfig.json is in the output folder of both the roslyn4.8 and the roslyn5.9 projects