Fixed constexpr append in FixedString tests
Fixed constexpr append in FixedString tests

Summary:
[Folly] Fixed constexpr append in `FixedString` tests after {D15618465}.

```
folly/test/FixedStringTest.cpp: In member function 'virtual void FixedStringAssignTest_ConstexprAppendLiteral_Test::TestBody()':
folly/test/FixedStringTest.cpp:360:3: error: non-constant condition for static assertion
   static_assert(constexpr_append_literal_test() == "hello world!", "");
   ^~~~~~~~~~~~~
In file included from folly/Range.h:25:0,
                 from folly/FixedString.h:33,
                 from folly/test/FixedStringTest.cpp:20:
folly/test/FixedStringTest.cpp:360:47:   in constexpr expansion of 'constexpr_append_literal_test()'
folly/test/FixedStringTest.cpp:355:18:   in constexpr expansion of '#'result_decl' not supported by dump_expr#<expression error>.folly::BasicFixedString<char, 20>::append((& s[7]))'
folly/FixedString.h:1277:48:   in constexpr expansion of 'folly::constexpr_strlen<char>(that)'
folly/portability/Constexpr.h:68:21: error: 'strlen((& s[7]))' is not a constant expression
   return std::strlen(s);
          ~~~~~~~~~~~^~~
```

Reviewed By: simpkins

Differential Revision: D15643402

fbshipit-source-id: c7e26a0c37c956300364c566f211c866985d9dc2

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