Include the executable extension in the test rule dependency

This fixes "make test" on platforms that have an executable
extension.
Do chdir within mktargets.py

This simplifies the calling shell script.
Merge pull request #203 from mstorsjo/make-dep-exeext

Include the executable extension in the test rule dependency
Merge pull request #209 from mstorsjo/chdir-within-mktargets

Do chdir within mktargets.py