[test] Move slow StringMemoryTest to validation-test (#18776)

This test takes over a minute to run, and all it's doing is testing
that repeated operations on a string don't leak.

validation-test/stdlib/StringMemoryTest.swift

@@ -0,0 +1,64 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-build-swift -O %s -o %t/StringMemoryTest
+// RUN: %target-codesign %t/StringMemoryTest
+// RUN: %target-run %t/StringMemoryTest | %FileCheck %s
+
+// REQUIRES: optimized_stdlib
+// REQUIRES: executable_test
+// REQUIRES: objc_interop
+
+import Foundation
+
+let str = "abcdefg\u{A758}hijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz\u{A759}"
+let str2 = "abcdefg\u{A759}hijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz\u{A758}"
+
+@inline(never)
+func lookup(_ str: String, _ dict: [String: Int]) -> Bool {
+  if let _ = dict[str] {
+    return true
+  }
+  return false
+}
+
+@inline(never)
+func uppercase(_ str: String) -> String {
+      return str.uppercased()
+}
+
+@inline(never)
+func lowercase(_ str: String) -> String {
+      return str.lowercased()
+}
+
+/// Make sure the hash function does not leak.
+
+let dict = [ "foo" : 1]
+for _ in 0 ..< 10_000_000 {
+  if lookup("\u{1F1E7}\u{1F1E7}", dict) {
+    print("Found?!")
+  }
+  if uppercase(str) == "A" {
+    print("Found?!")
+  }
+  if lowercase(str2) == "A" {
+    print("Found?!")
+  }
+}
+
+// CHECK-NOT: Found?!
+// CHECK: Not found
+
+print("Not found")
+
+var usage = rusage()
+getrusage(RUSAGE_SELF, &usage)
+
+// CHECK: success
+// CHECK-NOT: failure
+
+// We should not need 50MB for this.
+if usage.ru_maxrss > 50 * 1024 * 1024 {
+  print("failure - should not need 50MB!")
+} else {
+  print("success")
+}