RF: remove yields from test_mask
RF: remove yields from test_mask

The test generator makes it more difficult to debug; remove.

alexis-roche/nireg,alexis-roche/nireg,nipy/nireg,alexis-roche/nipy,alexis-roche/niseg,alexis-roche/nipy,alexis-roche/register,alexis-roche/nipy,arokem/nipy,alexis-roche/register,arokem/nipy,alexis-roche/nipy,alexis-roche/register,arokem/nipy,arokem/nipy,nipy/nireg,alexis-roche/niseg