tdb: add tests for double .close() in pytdb

tdb: add tests for double .close() in pytdb

The reason I do it is that when using older python-tdb as shipped in
Debian Lenny, python interpreter crashes on this test:

    (gdb) bt
    #0  0xb7f8c424 in __kernel_vsyscall ()
    #1  0xb7df5640 in raise () from /lib/i686/cmov/libc.so.6
    #2  0xb7df7018 in abort () from /lib/i686/cmov/libc.so.6
    #3  0xb7e3234d in __libc_message () from /lib/i686/cmov/libc.so.6
    #4  0xb7e38624 in malloc_printerr () from /lib/i686/cmov/libc.so.6
    #5  0xb7e3a826 in free () from /lib/i686/cmov/libc.so.6
    #6  0xb7b39c84 in tdb_close () from /usr/lib/libtdb.so.1
    #7  0xb7b43e14 in ?? () from /var/lib/python-support/python2.5/_tdb.so
    #8  0x0a038d08 in ?? ()
    #9  0x00000000 in ?? ()

master's pytdb does not (we have a check for self->closed in obj_close()),
but still...

Signed-off-by: Kirill Smelkov <kirr@mns.spb.ru>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>