fixed: linking due to several specializations being generated

due to the separate build steps for the library and the test binary, the current setup results in two copies of the specialization being generated, with the corresponding linker failure (below).

../../../../../lib/x86_64-linux-gnu/libParser.a(EclipseState.cpp.o): In function `Opm::GridProperty<int>::loadFromDeckKeyword(std::shared_ptr<Opm::DeckKeyword const>)':
EclipseState.cpp:(.text+0x0): multiple definition of `Opm::GridProperty<int>::loadFromDeckKeyword(std::shared_ptr<Opm::DeckKeyword const>)'
CMakeFiles/runEclipseStateTests.dir/EclipseStateTests.cpp.o:EclipseStateTests.cpp:(.text+0x220): first defined here
collect2: error: ld returned 1 exit status

i guess the msft linker have other rules here. this is perhaps not how you want it fixed, though it is a proper fix.