Address review comments on operator[](json_pointer) rollback

- undo() shrinks the array with resize() instead of erase()
- comments say what "will be" done in get_unchecked_create
- tests: avoid a clang-tidy performance-unnecessary-copy-initialization
  hit (unit-alt-string) and the word "access" in section names, which
  Flawfinder flags as race/access

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/src/unit-alt-string.cpp

@@ -385,9 +385,9 @@ TEST_CASE("alternative string type")
         CHECK(j.dump() == R"({"a":{"b":[1,null,null,{"x":[{"y":1}]},4],"z":2},"c":[{"q":3}]})");
 
         // a failing access leaves the value unchanged
-        alt_json k = before;
-        CHECK_THROWS_AS(k[alt_json::json_pointer("/a/n/99999999999999999999999")], alt_json::out_of_range&);
-        CHECK(k == before);
+        j = before;
+        CHECK_THROWS_AS(j[alt_json::json_pointer("/a/n/99999999999999999999999")], alt_json::out_of_range&);
+        CHECK(j == before);
     }
 
     SECTION("operator/(std::size_t)")

tests/src/unit-json_pointer.cpp

@@ -613,13 +613,13 @@ TEST_CASE("JSON pointers")
                                  "[json.exception.parse_error.106] parse error: array index '01' must not begin with '0'", json::parse_error&);
         }
 
-        SECTION("a failing access leaves the value unchanged")
+        SECTION("a failing operator[] leaves the value unchanged")
         {
             check_strong_guarantee_cases<json>();
             check_strong_guarantee_cases<nlohmann::ordered_json>();
         }
 
-        SECTION("a successful access still creates the same values")
+        SECTION("a successful operator[] still creates the same values")
         {
             json j = {{"a", {1}}};
             j["/a/3/x/-/y"_json_pointer] = 1;