Add parameter to ensure tests run in sequence regardless of when build completes

As initially reported in https://github.com/Microsoft/vstest/issues/1966, there isn’t currently a way to prevent dotnet test from running separate test assemblies in parallel when this is not desirable behaviour.

@ShreyasRmsft suggested that adding a new parameter to the cli that would ensure tests run in sequence regardless of when build completes would enable this.

[mikeharder]

Setting the MSBuild property BuildInParallel=false seems to work for dirs.proj (using Microsoft.Build.Traversal), though it doesn’t seem to work for *.sln files. Given the following dirs.proj:

<Project Sdk="Microsoft.Build.Traversal/2.0.24">
  <ItemGroup>
    <ProjectReference Include="**\*.*proj" />
  </ItemGroup>
</Project>

If you run dotnet test -p:BuildInParallel=false dirs.proj, each test project will be run in sequence. However, dotnet test -p:BuildInParallel=false foo.sln still executes test projects in parallel.

[baronfel]

The documentation for the -m option can be found on the MSBuild command-line reference. It controls the number of concurrent processes used to execute the build, so -m:1 would limit the build to one process.

More broadly, the dotnet build/test/restore/publish commands flow through many of these MSBuild parameters to the underlying invocation of MSBuild, which is why this works. There’s more documentation about this here.