Extend tests utility belt

[eiri]

It's be nice to have some standard test wrapper with 'retry until' or 'pass if any of N' functionality to deal with things like:

Compact after lowering attachment size limit couchdb_attachments_tests:544: should_compact_after_lowering_attachment_size_limit...*failed*

in function couchdb_attachments_tests:wait_compaction/3 (test/couchdb_attachments_tests.erl, line 586)
in call from couchdb_attachments_tests:compact_db/1 (test/couchdb_attachments_tests.erl, line 573)
in call from couchdb_attachments_tests:'-should_compact_after_lowering_attachment_size_limit/1-fun-1-'/1 (test/couchdb_attachments_tests.erl, line 550)

**error:{assertion_failed,[{module,couchdb_attachments_tests}, {line,573},
{reason,"Timeout waiting for database database compaction"}]}
  output:<<"">>

[janl]

@eiri problem is eunit standard timeouts. So even if we do a spinloop of “do until success”, we’d run into the eunit timeout. We can raise that of course, but we’ll be doing that a lot.

Maybe there is a way to set a timeout globally and set it to really long for travis and jenkins runs, so we never run into them™