recursion safety: enable doctests that call doctest

[cbm755]

Consider a test which itself calls doctest. E.g., if foo.m has:

@example
doctest chop
@print{} ...
@print{}     PASS     2/2
@end example

I get failures on doctest foo:

error: matrix cannot be indexed with {
error: called from
    doctest_datastore at line 38 column 9
    doctest_run_tests at line 73 column 25
    doctest_run_docstring at line 96 column 9
    doctest_collect at line 188 column 11
    doctest at line 289 column 11

I guess its the persistent variables not being safe in this way. I'm not losing sleep over this but it might be convenient within our own test suite.