Remove 'import *' statement from test/memorydb.py

It is generally best to avoid 'import *' statements, particularly in
this case where objects in the global namespace were then being
propagated through to an execfile(). The construction of the 'vars'
variable is a bit of a hack, but preferable to the alternative. Sometime
in the future it may be a good idea to do away with using execfile() to
parse the schema files.