Add regression test for FSFS limitation (and check) that only one file can be modified in TXN at time.
Add regression test for FSFS limitation (and check) that only one file can be
modified in TXN at time.

* subversion/tests/libsvn_fs/fs-test.c
  (test_modify_txn_being_written): New test.
  (test_funcs): Add test_modify_txn_being_written() test to list.

git-svn-id: f8a4e5e023278da1e04e203c7fe051e3c4285d88@1647773 13f79535-47bb-0310-9956-ffa450edef68

YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion