Set BuildAsStandalone for gcstress test builds

[markples]

No description provided.

[markples]

/azp run runtime-coreclr gcstress0x3-gcstress0xc

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[markples]

/azp run runtime-coreclr gcstress0x3-gcstress0xc

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[markples]

/azp run runtime-coreclr gcstress0x3-gcstress0xc

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[markples]

/azp run runtime-coreclr gcstress0x3-gcstress0xc

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[markples]

/azp run runtime-coreclr gcstress0x3-gcstress0xc

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[markples]

/azp run runtime-coreclr gcstress0x3-gcstress0xc

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[markples]

/azp run runtime-coreclr gcstress0x3-gcstress0xc

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[markples]

I'm going back to trying to get merging to work better (e.g., #85040). This has been painful and isn't there yet. There are a few fundamental points:

• A test needs to change between OutputType Exe vs Library for a merged test vs one with RequireProcessIsolation (or in BuildAsStandalone mode). While theoretically this shouldn't be different between RPI and BAS, I've hit a lot of issues messing around with it. This is how we've ended up in a situation where RPI works (only) when marked on tests and BAS works (only) as a global setting. This is relevant to the idea below of trying to run a few merged groups in a BAS build.
• We have some huge projects (such as JIT\HardwareIntrinsics\Arm\AdvSimd.Arm64\AdvSimd.Arm64_r.csproj). In merged test groups, the individual tests within this project can be striped to multiple helix jobs. In the old style, this test has a single entry point that runs everything, and it times out under gcstress.

I don't think investing in striping in BAS is the right direction as the main reason for BAS is to be simpler than merged groups, and I don't think investing in special cases in the msbuild BAS support is the right thing to do (or then maintain). Maybe after efforts fail again with merged groups I'll change my mind again.