only create svn reference repo once per data set

this improves speed dramatically since svn is super expensive.
goes from about 58 assertions/s to 133 a/s over an entire rake test run.

to facilitiate this there's some rejiggering in testme

• has new class extensions
• testdir and the derived datadir and data(path) are now class-level since they do not actually mutate with the individual instances but rather lookup fixtures based on the class type
• new assert_run on class and instance level to help prevent system() or `` failing and us not noticing/asserting this (specifically I suspect l10n may flake sometimes due to one of the setup calls falling off a cliff)

the way the actual setup works is by (ab)using ruby init behavior. consts
only get initialized once an so a block attached to a const is also only
run once per file effectively rendering it an only_once kind of affair.
on the opposite end of the life cycle sits the after_run hook which simply
runs after minitest is done with everything.

the actual setup() now simply copies the reference repo into place instead
of creating it from scratch every time