[ASTImporter] Refactor unittests to be able to parameterize them in a more flexible way

Summary:
Currently `TestImportBase` is derived from `ParameterizedTestsFixture`
which explicitly states that the gtest parameter can be only an
`ArgVector`. This is a limitation when we want to create tests which may
have different parameters.
E.g. we would like to create tests where we can combine different test
parameters. So, for example we'd like gtest to be able to provide
parameters of `<std::tuple<ArgVector, const char *>` instead of a simple
`ArgVector`.

Reviewers: a_sidorin, shafik, a.sidorin

Subscribers: rnkovacs, dkrupp, Szelethus, gamesh411, cfe-commits

Differential Revision: https://reviews.llvm.org/D57322

llvm-svn: 353425