[llvm] [NFC][ADT] Introduce a test harness for StringRefTest (PR #105500)

[Rahul Joshi via llvm-commits]

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@@ -47,37 +34,39 @@ static_assert(std::is_assignable_v<StringRef &, const char *&&>,
 static_assert(std::is_assignable_v<StringRef &, const char *&>,
               "Assigning from lvalue C string");
 
-namespace {
-TEST(StringRefTest, Construction) {
+// Test fixture.
+class StringRefTest : public testing::Test {};
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jurahul wrote:

Because that will come when I add test cases for reverse iterators, in a follow-on change. I assume I should not be rolling in this change into that. And I see other tests, like StringSetTest using a harness without any members.

https://github.com/llvm/llvm-project/pull/105500