Use fixtures / templates in LibLinear unit test

[karlnapf]

The unit tests for CLibLinear are highly redundant copy paste style code -- hard to maintain/read/change.
https://github.com/shogun-toolbox/shogun/blob/develop/tests/unit/classifier/svm/LibLinear_unittest.cc

This easy entrance task is to use googletest's fixture concept to reduce the number of lines of code in this test. It should be possible to reduce things down to 1-2 explicitly coded out tests.

Useful ressources

• Grep for TEST_F in the shogun tests
• https://github.com/google/googletest/blob/master/googletest/docs/Primer.md (search TEST_F)
• https://github.com/google/googletest/blob/master/googletest/docs/AdvancedGuide.md (search TYPED_TEST)

[vigsterkr]

closing as this has been fixed