Revert "Reenable exit tests on Ubuntu 20.04 (swiftlang#1343)"

This reverts commit a883652.

Author: jmschonfeld

Package.swift

@@ -73,11 +73,20 @@ let wasiLibcCSettings: [CSetting] = [
     .define("_WASI_EMULATED_MMAN", .when(platforms: [.wasi])),
 ]
 
-let testOnlySwiftSettings: [SwiftSetting] = [
+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")],