Make has a builtin operator 'undefine', and coupled with latest
automake.git, this test name ended up confusing make into thinking
the file name was meant to be used as the make operator. Renaming
the file avoids the confusion.
* tests/undefine: Rename...
* tests/virsh-undefine: ...to this.
* tests/Makefile.am (test_scripts): Use new name.
Reported by Jim Meyering.
