Ensure Xcode SDK paths are set when running test targets. (#7093)

Currently, some of our code paths that run test targets don't correctly
set the `DYLD_FRAMEWORK_PATH` and `DYLD_LIBRARY_PATH` arguments on
macOS. This hasn't been a problem in practice because the XCTest helper
tool is able to sort it out for us, but my plan with swift-testing is to
run an executable directly (like we do for corelibs-xctest.) That means
we'll have a code path that could potentially rely on XCTest or other
Apple SDK bits, but which is not calling through the XCTest helper tool.

This change hoists our setting of those arguments to a position where
they are always set (on macOS only!) before we run a test target,
regardless of build style.

This change was previously landed as #7040 but was reverted (#7054)
while we investigated a toolchain regression. We do not believe #7040 is
the root cause of that issue, so this PR reapplies it.

Author: grynspan

Sources/Commands/Utilities/TestingSupport.swift

@@ -108,7 +108,7 @@ enum TestingSupport {
         #if os(macOS)
         let data: String = try withTemporaryFile { tempFile in
             args = [try Self.xctestHelperPath(swiftTool: swiftTool).pathString, path.pathString, tempFile.path.pathString]
-            var env = try Self.constructTestEnvironment(
+            let env = try Self.constructTestEnvironment(
                 toolchain: try swiftTool.getTargetToolchain(),
                 buildParameters: swiftTool.buildParametersForTest(
                     enableCodeCoverage: enableCodeCoverage,
@@ -118,12 +118,6 @@ enum TestingSupport {
                 sanitizers: sanitizers
             )
 
-            // Add the sdk platform path if we have it. If this is not present, we might always end up failing.
-            let sdkPlatformFrameworksPath = try SwiftSDK.sdkPlatformFrameworkPaths()
-            // appending since we prefer the user setting (if set) to the one we inject
-            env.appendPath("DYLD_FRAMEWORK_PATH", value: sdkPlatformFrameworksPath.fwk.pathString)
-            env.appendPath("DYLD_LIBRARY_PATH", value: sdkPlatformFrameworksPath.lib.pathString)
-
             try TSCBasic.Process.checkNonZeroExit(arguments: args, environment: env)
             // Read the temporary file's content.
             return try swiftTool.fileSystem.readFileContents(AbsolutePath(tempFile.path))
@@ -180,6 +174,12 @@ enum TestingSupport {
         #endif
         return env
         #else
+        // Add the sdk platform path if we have it. If this is not present, we might always end up failing.
+        let sdkPlatformFrameworksPath = try SwiftSDK.sdkPlatformFrameworkPaths()
+        // appending since we prefer the user setting (if set) to the one we inject
+        env.appendPath("DYLD_FRAMEWORK_PATH", value: sdkPlatformFrameworksPath.fwk.pathString)
+        env.appendPath("DYLD_LIBRARY_PATH", value: sdkPlatformFrameworksPath.lib.pathString)
+
         // Fast path when no sanitizers are enabled.
         if sanitizers.isEmpty {
             return env