Avoid running the same test multiple times.
Avoid running the same test multiple times.

Previously nosetests searches for all the nosetest files in the
current directory and its subdirectories. That causes duplication
when we try to add_nosetests() on different files. Now change
add_nosetests() to accept a filename prefix. This also makes the
test more explicit.

dneto0/shaderc,AWoloszyn/shaderc,fuchsia-mirror/third_party-shaderc,Qining/shaderc,dneto0/shaderc,AWoloszyn/shaderc,fuchsia-mirror/third_party-shaderc,Qining/shaderc,antiagainst/shaderc,dneto0/shaderc,AWoloszyn/shaderc,antiagainst/shaderc,antiagainst/shaderc,fuchsia-mirror/third_party-shaderc,dneto0/shaderc,Qining/shaderc,antiagainst/shaderc