test.cc: find minimal.mp4 relative to the executable.
test.cc: find minimal.mp4 relative to the executable.

test_arg_validation_with_data() looked for 'examples/minimal.mp4'
which only worked when invoked from the top-level source directory.
In particular, it broke `make -C examples check` or plain
`make check` inside the examples directory.

Look for minimal.mp4 in the same directory path as the executable
itself so the test works regardless the working directory when
it's invoked.

kinetiknz/mp4parse-rust,mozilla/mp4parse-rust,kinetiknz/mp4parse-rust