Moving the stuff to a t subdirectory, "nmake test" tells me that M1 etc. are not found. Apparently it does not chdir to the t directory before running the tests. How do I say in the Makefile.PL that the modules needed for the test scripts are also in that directory?

There isn't a way of doing this in the Makefile.PL that I am aware of. (Or, to be more accurate, no standard way. You can always write some custom code.)

People normally add the directory to @INC in the appropriate *.t files.

Also, what is the significance of the name 00-basic

Adding a number prefix is one way to define the order test scripts are run in.

Test are run in filename order. Some people like to have their test scripts run in a certain order. For example, you might test some basic stuff in the first script run and then bail out of the test suite if it fails - saving time when something goes wrong.