Add PYBIND11_CATCH2_SKIP_IF macro to skip tests at runtime

Catch2 v2 doesn't have native skip support (v3 does with SKIP()).
This macro allows tests to be skipped with a visible message while
still appearing in the test list.

Use this for the Move Subinterpreter test on free-threaded Python 3.14+
so it shows as skipped rather than being conditionally compiled out.

Example output:
  [ RUN      ] Move Subinterpreter
  [ SKIPPED ] Skipped on free-threaded Python 3.14+ (see PR pybind#5940)
  [       OK ] Move Subinterpreter

Author: rwgk

tests/test_with_catch/catch.cpp

@@ -25,6 +25,8 @@ PYBIND11_WARNING_DISABLE_MSVC(4996)
 
 #define CATCH_CONFIG_RUNNER
 #define CATCH_CONFIG_DEFAULT_REPORTER "progress"
+#include "catch_skip.h"
+
 #include <catch.hpp>
 
 namespace py = pybind11;

tests/test_with_catch/catch_skip.h

@@ -0,0 +1,16 @@
+// Macro to skip a test at runtime with a visible message.
+// Catch2 v2 doesn't have native skip support (v3 does with SKIP()).
+// The test will count as "passed" in totals, but the output clearly shows it was skipped.
+
+#pragma once
+
+#include <catch.hpp>
+
+#define PYBIND11_CATCH2_SKIP_IF(condition, reason)                                                \
+    do {                                                                                          \
+        if (condition) {                                                                          \
+            Catch::cout() << "[ SKIPPED  ] " << reason << '\n';                                   \
+            Catch::cout().flush();                                                                \
+            return;                                                                               \
+        }                                                                                         \
+    } while (0)

tests/test_with_catch/test_subinterpreter.cpp

@@ -6,6 +6,8 @@
 // catch 2.0.1; this should be fixed in the next catch release after 2.0.1).
 PYBIND11_WARNING_DISABLE_MSVC(4996)
 
+#    include "catch_skip.h"
+
 #    include <catch.hpp>
 #    include <cstdlib>
 #    include <fstream>
@@ -90,11 +92,15 @@ TEST_CASE("Single Subinterpreter") {
     unsafe_reset_internals_for_single_interpreter();
 }
 
-// "Move Subinterpreter" test is disabled on free-threaded Python 3.14+ due to a hang
-// in Py_EndInterpreter() when the subinterpreter is destroyed from a different thread
-// than it was created on. See: https://github.com/pybind/pybind11/pull/5940
-#    if PY_VERSION_HEX >= 0x030D0000 && !(PY_VERSION_HEX >= 0x030E0000 && defined(Py_GIL_DISABLED))
+#    if PY_VERSION_HEX >= 0x030D0000
 TEST_CASE("Move Subinterpreter") {
+    // Test is skipped on free-threaded Python 3.14+ due to a hang in Py_EndInterpreter()
+    // when the subinterpreter is destroyed from a different thread than it was created on.
+    // See: https://github.com/pybind/pybind11/pull/5940
+#        if PY_VERSION_HEX >= 0x030E0000 && defined(Py_GIL_DISABLED)
+    PYBIND11_CATCH2_SKIP_IF(true, "Skipped on free-threaded Python 3.14+ (see PR #5940)");
+#        endif
+
     std::unique_ptr<py::subinterpreter> sub(new py::subinterpreter(py::subinterpreter::create()));
 
     // on this thread, use the subinterpreter and import some non-trivial junk