Unbreak test_dbfn make-target

doc/dbm.discuss.txt describes how to make and use `test_dbfn` for
testing DB functionality.

Commit cf0812d5 adds a call to assert_no_variables into store.c which
depends upon expand.c functionality and we can't link that in for
test_dbfn without pulling in half of Exim.

So adjust the test_dbfn target to rebuild store.o in COMPILE_UTILITY
mode and link against that variant, then remove the custom-built store.o
after the executable has been linked.