Functions under test are dumpSharedLocalSnapshot_forCursor() and
readSharedLocalSnapshot_forCursor().  Significant amount of global state needs
to be created for the test to be able to invoke the two functions.  What we are
testing here is, whether correct snapshot information is included in what is
written to file.  Validation is performed by reading the contents of the file
and compairing them with expected values.

An implicit rule to run a unittest is defined in src/Makefile.mock.  This patch
overrides it such that the required directory for the cursor snapshot file is
created before running the test.

Reviewed-by: Adam Berlin and Jesse Zhang