Summary:
I split the db-specific test points out into a separate file under db/
directory. There were also a few bugs to fix in xfunc.{h,cc} that prevented it
from compiling previously; see https://reviews.facebook.net/D36825.
Test Plan:
compilation works now, below command works, will also run "make xfunc".
$ make check ROCKSDB_XFUNC_TEST='managed_new' tests-regexp='DBTest' -j32
Reviewers: sdong, yhchiang
Subscribers: dhruba
Differential Revision: https://reviews.facebook.net/D53343
