Fixes #150.

The `mxwrite` example program previously hard-coded its output to
`./example.musicxml`, which left an untracked file at the repo root
every time examples ran (i.e. for every `make test` / `make test-all`
invocation).

## Changes

- **`src/private/mx/examples/Write.cpp`** \u2014 `mxwrite` now accepts
an
  optional output path via `argv[1]`, defaulting to
  `./example.musicxml` so the README's illustrative path still works
  when a user runs the binary by hand.
- **`Makefile`** \u2014 `run_examples` now passes
  `./data/testOutput/example.musicxml` to `mxwrite` and ensures the
  directory exists. `data/testOutput/` is already gitignored.
- **`data/testOutput/.gitignore`** \u2014 added `*.musicxml` (it only
  covered `*.xml`, `*.csv`, `*.txt` before).
- **`src/private/mxtest/api/DocumentManagerTest.cpp`** and
**`src/private/mxtest/api/RoundTrip.h`** \u2014 two other `writeToFile`
  calls that also dumped XML at the repo root (`sillytest.xml` and
  `output.xml`) now write into `data/testOutput` via
  `mxtest::getResourcesDirectoryPath()`.
- **`README.md`** \u2014 the example code snippet matches the updated
  `Write.cpp`.

## Verification

Ran locally:

- `make fmt` \u2014 clean
- `make check` \u2014 `=== check passed ===`
- `make test-all` \u2014 `All tests passed (9914 assertions in 2678 test
cases)`

After the full run, no `example.musicxml`, `example.xml`,
`sillytest.xml`, or `output.xml` remain at the repo root.

This supersedes #151.