STY: style cleanups for test_field
STY: style cleanups for test_field

Clean whitespace.
Reorganize imports.
Replace assert something with assert_true(something) and
assert_equal(something, something_else).
Replace docstrings for test functions with comments so we can see which
test is running during a default test run.

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