listruns succeeds even when number of existing runs is less than requested

[prjemian]

fixes #330

[prjemian]

travis ran successfully but py36 build reported this error:

======================================================================

ERROR: test_listruns (tests.test_utils.Test_With_Database)

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Traceback (most recent call last):

  File "tests/../tests/test_utils.py", line 245, in test_listruns

    db=self.db,

  File "tests/../tests/../apstools/utils.py", line 324, in listruns

    runs = db[-min(abs(num), len(db.v2.items())):]

TypeError: object of type 'generator' has no len()

[prjemian]

All travis builds (py36, py37, py38) reported this error.

Why a reported error from testing but travis reports success?