Simplify Test target to not create stub Directory.Build.* files

[mthalman]

See the comment at #85 (comment)

[akoeplinger]

I'm not actually sure this is better/preferred. If you set ImportDirectoryBuildProps=false it disables importing Directory.Build.props completely e.g. even if you have it in a subdirectory. With the tests we just want to prevent importing the D.B.props outside of the test directory so having an empty one seems better to me.