Adding tests for files with multiple "smpl" chunks

"sample_chunk" -> "smpl_chunk" in fixture names

Using better field value in fixtures

Using reference data instead of hard-coded value

Removing unneeded ".rb" suffixes in `require`

Removing duplication in reference sample data

Fixing order of values in assertion

Closing `Reader` instances in tests, for hygiene

Adding test assertions for consistency

Removing mismatch between variable name and type

Removing duplicated logic for building file names

Moving test method to more specific location

Adding extra test case for `Writer#total_duration`

Improving test suite performance 1.68x

Reading sample data in more flexible way in tests