VariantField: fix operator[] skip size

it was skipping the number of samples when it should be skipping the
number of **values per sample**.

fixes #146

A few updates to the test routine to avoid code duplication and catch
this bug should it ever happen again:
 * VariantReaderTest: make truth data globally available
 * VariantReaderTest: move multiple variant reader test code into variant reader test for reuse
 * Add variable values to the GQ tests
TODO: a test for the MultipleVariantReader that uses multiple files that are not actually the same (the current test uses the same file, just in different formats)