[libc++][test] Restore several variant tests

[fsb4000]

Summary

They were removed in https://reviews.llvm.org/D120684
I think it was made by mistake...
The code compiles just fine, even with -Weverything: https://gcc.godbolt.org/z/oKjvb8oPx

[fsb4000]

Oh, it seems it fails with gcc and libc++ and -D_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER
I use clang-cl and Windows locally so I can't test it locally but now I think the tests were removed intentionally.
Ok.

libcxx/test/std/utilities/variant/variant.variant/variant.assign/T.pass.cpp

#endif
     assert(std::get<0>(v) == "bar");
   }
   {
     std::variant<bool, std::unique_ptr<int>> v;
     v = nullptr;
     assert(v.index() == 1);
     assert(std::get<1>(v) == nullptr);
   }
+  {
+    std::variant<bool volatile, int> v = 42;
+    v = false;
+    assert(v.index() == 0);
+    assert(!std::get<0>(v));
+    bool lvt = true;
+    v = lvt;
+    assert(v.index() == 0);
+    assert(std::get<0>(v));
+  }
 #if !defined(TEST_VARIANT_HAS_NO_REFERENCES)
   {
     using V = std::variant<int &, int &&, long>;
     int x = 42;
     V v(43l);
     v = x;
     assert(v.index() == 0);
     assert(&std::get<0>(v) == &x);