This is to match the layout of the installed and source files more closely.
While doing this, honor the MKSHARE variable, as the files these tests
validate are only installed when MKSHARE=yes.
The main goal of this change is to simplify the further addition of test
cases for the share/mk infrastructure by adding a few placeholder test
programs to the tests tree.
To not leave these test programs empty, I have added a bunch of extremely
simple test cases to them.