Move SWIFT_TESTING_ENABLED to the right location

Author: grynspan

Sources/Commands/Utilities/TestingSupport.swift

@@ -189,6 +189,11 @@ enum TestingSupport {
             env.appendPath(key: "DYLD_LIBRARY_PATH", value: sdkPlatformFrameworksPath.lib.pathString)
         }
 
+        // We aren't using XCTest's harness logic to run Swift Testing tests.
+        if library == .xctest {
+            env["SWIFT_TESTING_ENABLED"] = "0"
+        }
+
         // Fast path when no sanitizers are enabled.
         if sanitizers.isEmpty {
             return env
@@ -205,10 +210,6 @@ enum TestingSupport {
         }
 
         env["DYLD_INSERT_LIBRARIES"] = runtimes.joined(separator: ":")
-
-        if library == .xctest {
-            env["SWIFT_TESTING_ENABLED"] = "0"
-        }
         return env
         #endif
     }