Turn the #972 fifo_map regression test into a real test

The #972 regression test in unit-regression1.cpp only built a
my_json array from a string literal (the original crash) and had no
CHECK, so the fifo_map object type it exists to demonstrate was never
exercised. Meanwhile the docs recommend fifo_map for keeping object
keys in insertion order (object_order.md, template_parameters.md),
and nothing tested that recommendation.

Extend the section: after the original array assignment, parse an
object with my_json::parse() (not via the "..."_json UDL, which
returns a plain nlohmann::json and would exercise the cross-basic_json
conversion constructor instead of the parser's own key insertion -
and, as tried locally, does not keep fifo order for this stateful
comparator) and check that dump() keeps insertion order, and that it
survives erase() and inserting a new key.

Also narrow thirdparty/fifo_map off the include path of every other
test-* target: it was a PUBLIC include directory of test_main, even
though unit-regression1.cpp is its only user. Add a small
fifo_map_include INTERFACE library with that include directory and
attach it to test-regression1 only via json_test_set_test_options().

Verified locally (test-regression1_cpp11, default build and
-fsanitize=address,undefined): the new checks pass; `git grep fifo_map
tests` still only finds unit-regression1.cpp and the vendored header.

Closes #5714 item 7.

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

tests/src/unit-regression1.cpp

@@ -1482,7 +1482,23 @@ TEST_CASE("regression tests 1")
 
     SECTION("issue #972 - Segmentation fault on G++ when trying to assign json string literal to custom json type")
     {
+        // this assignment used to crash outright
         my_json const foo = R"([1, 2, 3])"_json;
+
+        // fifo_map is the adapter the docs recommend for keeping object keys
+        // in insertion order (see docs/mkdocs/docs/features/object_order.md
+        // and docs/mkdocs/docs/features/types/template_parameters.md); check
+        // that recommendation actually holds, including through erase() and
+        // inserting a new key. The comparator is stateful, so this avoids
+        // deep copies of "order" (see #1763, #5649).
+        my_json order = my_json::parse(R"({"z":1,"a":2,"m":{"y":1,"b":2}})");
+        CHECK(order.dump() == R"({"z":1,"a":2,"m":{"y":1,"b":2}})");
+
+        order.erase("z");
+        CHECK(order.dump() == R"({"a":2,"m":{"y":1,"b":2}})");
+
+        order["new_key"] = 3;
+        CHECK(order.dump() == R"({"a":2,"m":{"y":1,"b":2},"new_key":3})");
     }
 
     SECTION("issue #977 - Assigning between different json types")