[libc++] Refactor string unit tests to ease addition of new allocators

While doing this, I also found a few tests that were either clearly
incorrect (e.g. testing the wrong function) or that lacked basic test
coverage like testing std::string itself (e.g. the test was only checking
std::basic_string with a custom allocator). In these cases, I did a few
conservative drive-by changes.

Differential Revision: https://reviews.llvm.org/D140550
Co-authored-by: Brendan Emery <brendan.emery@esrlabs.com>

libcxx/test/std/strings/basic.string/string.modifiers/string_assign/string.pass.cpp

@@ -90,7 +90,9 @@ TEST_CONSTEXPR_CXX20 bool test() {
     testAlloc(S(A(5)), S("1"), A());
     testAlloc(S(A(5)), S("1", A(7)), A(7));
     testAlloc(S(A(5)), S("1234567890123456789012345678901234567890123456789012345678901234567890", A(7)), A(7));
-    testAlloc(S("12345678901234567890", A(5)), S("1234567890123456789012345678901234567890123456789012345678901234567890", A(7)), A(7));
+    testAlloc(S("12345678901234567890", A(5)),
+              S("1234567890123456789012345678901234567890123456789012345678901234567890", A(7)),
+              A(7));
   }
 
 #if TEST_STD_VER >= 11