py:dcerpc/tests: rename dcerpc/string.py -> string_tests.py

py:dcerpc/tests: rename dcerpc/string.py -> string_tests.py

Otherwise it's not possible to run the raw_protocol tests anymore:

python/samba/tests/dcerpc/raw_protocol.py
Traceback (most recent call last):
  File "python/samba/tests/dcerpc/raw_protocol.py", line 26, in <module>
    import samba.dcerpc.dcerpc as dcerpc
  File "bin/python/samba/__init__.py", line 32, in <module>
    from samba.compat import string_types
  File "bin/python/samba/compat.py", line 151, in <module>
    from urllib import quote as urllib_quote
  File "/usr/lib/python2.7/urllib.py", line 25, in <module>
    import string
  File "/abs/path/samba/python/samba/tests/dcerpc/string.py", line 22, in <module>
    # Some strings for ctype-style character classification
  File "bin/python/samba/tests/__init__.py", line 36, in <module>
    from samba.compat import text_type
ImportError: cannot import name text_type

This allows the following again:

SMB_CONF_PATH=/dev/null \
    SERVER=172.31.9.188 \
    TARGET_HOSTNAME=w2012r2-188.w2012r2-l6.base \
    USERNAME=administrator \
    PASSWORD=A1b2C3d4 \
    DOMAIN=W2012R2-L6 \
    REALM=W2012R2-L6.BASE \
    python/samba/tests/dcerpc/raw_protocol.py -v -f TestDCERPC_BIND

BUG: https://bugzilla.samba.org/show_bug.cgi?id=7113
BUG: https://bugzilla.samba.org/show_bug.cgi?id=11892

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>