Increase test coverage

1. Increase test coverage.
2. Use `rsmt2d_test` package to test all exported methods.
3. Consider using `rsmt2d_test` test package for more of the tests, to not rely on internal methods as much.

Start by reviewing the exported methods in the rsmt2d package and the existing tests, then inspect the rsmt2d_test package boundary. Add coverage for all exported methods and move suitable tests away from internal methods where practical; done means the requested public API behavior is covered by tests.

Written by the indexing model from the issue text.