fix coverage break gevent/eventlet testcase

The coverage.process_starup() will import threading which breaks gevent/eventlet monkey patch

Before the patch
```
python test/test_server.py
WARNING: Skipping 'meinheld' test (ImportError).
.WARNING: Skipping 'bjoern' test (ImportError).
.WARNING: Skipping 'cherrypy' test (ImportError).
.WARNING: Skipping 'diesel' test (ImportError).
..WARNING: Skipping 'fapws3' test (ImportError).
.E..WARNING: Skipping 'rocket' test (ImportError).
....
======================================================================
ERROR: test_simple (__main__.TestGeventServer)
Test a simple static page with this server adapter.
----------------------------------------------------------------------
Traceback (most recent call last):
  File "test/test_server.py", line 79, in tearDown
    raise AssertionError(line.strip().decode('utf8'))
AssertionError: Exception KeyError: KeyError(139741601585264,) in <module 'threading' from '/usr/lib/python2.7/threading.pyc'> ignored

----------------------------------------------------------------------
Ran 13 tests in 6.463s

FAILED (errors=1)
```
After this PR
```
python test/test_server.py
WARNING: Skipping 'meinheld' test (ImportError).
.WARNING: Skipping 'bjoern' test (ImportError).
.WARNING: Skipping 'cherrypy' test (ImportError).
.WARNING: Skipping 'diesel' test (ImportError).
..WARNING: Skipping 'fapws3' test (ImportError).
....WARNING: Skipping 'rocket' test (ImportError).
....
----------------------------------------------------------------------
Ran 13 tests in 6.463s

OK
```