Warnings from new tests

[jessemapel]

I get the following warnings from our new tests when I build:

/usgs/pkgs/isis3nightly2018-11-27/isis/tests/PixelTests.cpp:468:3: warning: floating constant exceeds range of ‘double’ [-Woverflow]
   EXPECT_EQ(std::string("Invalid"), Pixel::ToString(-1.0e+1000));
   ^
/usgs/pkgs/isis3nightly2018-11-27/isis/tests/PixelTests.cpp:480:3: warning: floating constant exceeds range of ‘double’ [-Woverflow]
   EXPECT_EQ(std::string("Invalid"), Pixel(1,2,3,-1.0e+1000).ToString());
   ^