Added more test cases

* Added tests for the following files from
  matio_test_dataset (http://sourceforge.net/projects/matio/)

  - inst/tests/matio_test_cases_compressed_le.mat
  - inst/tests/matio_test_cases_v4_be.mat
  - inst/tests/matio_test_cases_v4_le.mat
  - inst/tests/small_v4_be.mat
  - inst/tests/small_v4_le.mat