[Piglit] framework: don't allow commands to contain spaces in the elements

[Dylan Baker]

Python's subprocess.Popen command takes a list as an argument, and
elements should not contain spaces. But we don't enforce that behavior,
it results in odd failures when it does. This series adds a check to
ensure that it doesn't, and errors out if it does find one.

The first 3 patches fix errors found by patch 4