Fix Optional test for -fb platform

Summary:
The Optional test relied on std::string clearing its data when
it is the source of a move. This does not happen for in-situ fbstrings,
so the test breaks in the -fb platform. The solution: wrap the string in
a class that explicitly invalidates its data upon a move.

Test Plan:
fbconfig --platform-all=gcc-4.8.1-glibc-2.17-fb -r folly
fbmake runtests
fbconfig -r folly
fbmake runtests

Reviewed By: tudorb@fb.com

Subscribers: sdwilsh

FB internal diff: D1506840

Tasks: 4943996

folly/test/OptionalTest.cpp

@@ -108,9 +108,28 @@ TEST(Optional, Simple) {
   EXPECT_FALSE(bool(opt));
 }
 
+class MoveTester {
+public:
+  /* implicit */ MoveTester(const char* s) : s_(s) {}
+  MoveTester(const MoveTester&) = default;
+  MoveTester(MoveTester&& other) noexcept {
+    s_ = std::move(other.s_);
+    other.s_ = "";
+  }
+  MoveTester& operator=(const MoveTester&) = default;
+  MoveTester& operator=(MoveTester&&) = default;
+private:
+  friend bool operator==(const MoveTester& o1, const MoveTester& o2);
+  std::string s_;
+};
+
+bool operator==(const MoveTester& o1, const MoveTester& o2) {
+  return o1.s_ == o2.s_;
+}
+
 TEST(Optional, value_or_rvalue_arg) {
-  Optional<std::string> opt;
-  std::string dflt = "hello";
+  Optional<MoveTester> opt;
+  MoveTester dflt = "hello";
   EXPECT_EQ("hello", opt.value_or(dflt));
   EXPECT_EQ("hello", dflt);
   EXPECT_EQ("hello", opt.value_or(std::move(dflt)));