abc2midi: allow %%MIDIdef macros to be defined anywhere (#23)

* abc2midi: allow %%MIDIdef macros to be defined anywhere

Fix also some lost CHANGES from previous commit.

abc2midi: allow %%MIDIdef macros to be defined anywhere, including before the first X: field (outside any tune). Previously %%MIDIdef was only handled inside event_specific() after the started_parsing check,so macros defined in the file header were silently ignored.
The %%MIDIdef handling has been moved before the started_parsing check in event_specific() in store.c since macro definitions are context-independent.

* Update the golden test files since X:, R: and C: are now emitted

README.md

@@ -61,3 +61,30 @@ Available presets:
 The CMake build generates `compile_commands.json` for use with
 clangd and other LSP-based editors.
 
+### Testing
+
+The CMake build includes a test suite covering all 8 programs:
+
+- **Smoke tests** verify each binary runs cleanly with `-ver`.
+- **Golden-file tests** run each program on a sample input and compare the
+  (normalized) output to a checked-in reference. Binary MIDI outputs are
+  piped through `mftext` to produce diffable text. Volatile lines (version
+  banners, dates, temporary paths) are stripped before comparison.
+
+```sh
+# Run all tests
+ctest --preset debug
+
+# Run only golden-file tests / only smoke tests
+ctest --preset debug -L golden
+ctest --preset debug -L smoke
+```
+
+To regenerate the golden files after an intentional behavioural change,
+review the diff, then commit:
+
+```sh
+cmake --build build/debug --target update-golden
+git diff tests/golden/
+```
+