[5.9] Reduce XCTest minimum deployment target computation

[neonichu]

We never need this for any platform that we're not building for and we also don't really need it for most commands. So we can just move the computation to SwiftTool and leave these empty for all other cases.

rdar://64596106

(cherry picked from commit 019a6fb)

[neonichu]

@swift-ci please test

[neonichu]

error: the test 'IntegrationTests' requires macos 10.13, but depends on the product 'SwiftToolsSupport-auto' which requires macos 10.15; consider changing the test 'IntegrationTests' to require macos 10.15 or later, or the product 'SwiftToolsSupport-auto' to require macos 10.13 or earlier.
error: the test 'IntegrationTests' requires macos 10.13, but depends on the product 'TSCTestSupport' which requires macos 10.15; consider changing the test 'IntegrationTests' to require macos 10.15 or later, or the product 'TSCTestSupport' to require macos 10.13 or earlier.

The first time I saw this I took it at face value and updated the integration tests package, but I think this is actually an issue with this PR. The diagnostic is technically correct in the sense that these are the deployment targets at the package level, but the test target specifically should receive a raised deployment target of macOS 11 or maybe even 12?

[neonichu]

Hm, it doesn't seem as if I can reproduce the IntegrationTests issue locally

[neonichu]

Ah, I cannot reproduce locally because the Xcode I am using contains XCTest with a macOS 13 deployment target which is new enough to fulfill the TSC requirement. So that suggests that the raising of the deployment target still works correctly with my changes, at least in principle.

So that means it is correct to update the IntegrationTests package manifest, but I still cannot explain why the issue is only surfacing in this PR. Seems like there must still be a change in behavior somewhere, albeit it seems like that might bring us closer to expect behavior.

[neonichu]

Scratch that, this was working locally because I had a random old resolved file lying around in the IntegrationTests, so I was getting a version that still used a 10.13 minimum deployment target.

[neonichu]

Found the issue:

diff --git a/Sources/Build/BuildPlan.swift b/Sources/Build/BuildPlan.swift
index 1799c963f..b5873c7ca 100644
--- a/Sources/Build/BuildPlan.swift
+++ b/Sources/Build/BuildPlan.swift
@@ -573,7 +573,7 @@ public class BuildPlan: SPMBuildCore.BuildPlan {
         // Supported platforms are defined at the package level.
         // This will need to become a bit complicated once we have target-level or product-level platform support.
         let productPlatform = product.platforms.getDerived(for: .macOS, usingXCTest: product.isLinkingXCTest)
-        let targetPlatform = target.platforms.getDerived(for: .macOS, usingXCTest: product.isLinkingXCTest)
+        let targetPlatform = target.platforms.getDerived(for: .macOS, usingXCTest: target.type == .test)
 
         // Check if the version requirement is satisfied.
         //

[neonichu]

@swift-ci please test

[neonichu]

@swift-ci please test windows

[neonichu]

We'll need the fix in main as well and should revert the changes to IntegrationTests there.