kokkos: refactor duplicated test helpers

[mzuzek]

This PR moves common Kokkos test utilities to shared headers - refactoring massive duplication and significantly shortening test files.

[mhoemmen]

Awww, no complex_float?

[fnrizzi]

That is my fault because I started all tests using double, float and complex<double>, so Miko was following my setup :)
I created a new issue: #190 to add complex<float> later

[fnrizzi]

@MikolajZuzek my preference would be to remove this macro. It is just 3 lines of code :) it does not save you much typing. Also, I think it adds another layer that one would have to find when looking at the tests.
I think this gtest_fixtures should only contain the fixtures. If you really want this macro, maybe we can put it in another file called test_macros.hpp or similar so that it is clear the distinction.

[mzuzek]

Macro removed on 6be6990. It seemed like a good idea couple gtest_fixtures.hpp types in a macro and place it close by, but maybe we can just remember to keep that synchronized in test files.

[crtrott]

Certainly a good improvement. But where is this _blas2_signed_fixture thing coming from? Is that some semi pulic non-namespaced thing in our headers?