Reenable AttributedString index tracking swift testing tests (#1338)

* Revert "Revert "Add tests for AttributedString Index Tracking preconditions (#1326)" (#1337)"

This reverts commit 2df5199.

* Disable exit tests on Ubuntu 20.04 for now

Author: jmschonfeld

Package.swift

@@ -75,6 +75,20 @@ let wasiLibcCSettings: [CSetting] = [
     .define("_WASI_EMULATED_MMAN", .when(platforms: [.wasi])),
 ]
 
+var testOnlySwiftSettings: [SwiftSetting] = [
+    // The latest Windows toolchain does not yet have exit tests in swift-testing
+    .define("FOUNDATION_EXIT_TESTS", .when(platforms: [.macOS, .linux, .openbsd]))
+]
+
+#if os(Linux)
+import FoundationEssentials
+
+if ProcessInfo.processInfo.operatingSystemVersionString.hasPrefix("Ubuntu 20.") {
+    // Exit tests currently hang indefinitely on Ubuntu 20.
+    testOnlySwiftSettings.removeFirst()
+}
+#endif
+
 let package = Package(
     name: "swift-foundation",
     platforms: [.macOS("15"), .iOS("18"), .tvOS("18"), .watchOS("11")],
@@ -172,7 +186,7 @@ let package = Package(
                 "LifetimeDependenceMutableAccessors",
                 .when(platforms: [.macOS, .iOS, .watchOS, .tvOS, .linux])
               ),
-            ] + availabilityMacros + featureSettings
+            ] + availabilityMacros + featureSettings + testOnlySwiftSettings
         ),
 
         // FoundationInternationalization
@@ -206,7 +220,7 @@ let package = Package(
                 "TestSupport",
                 "FoundationInternationalization",
             ],
-            swiftSettings: availabilityMacros + featureSettings
+            swiftSettings: availabilityMacros + featureSettings + testOnlySwiftSettings
         ),
 
         // FoundationMacros
@@ -238,7 +252,7 @@ package.targets.append(contentsOf: [
             "FoundationMacros",
             "TestSupport"
         ],
-        swiftSettings: availabilityMacros + featureSettings
+        swiftSettings: availabilityMacros + featureSettings + testOnlySwiftSettings
     )
 ])
 #endif