Following up on r1619105, tweak the situations when we would like to skip
the new revprop_caching_on_off() test and its expected outcome.

* subversion/tests/libsvn_fs_fs/fs-fs-pack-test.c
  (ignore_fs_warnings): New.
  (revprop_caching_on_off): Tweak the conditions under which we want to skip
    this test — now we execute it unconditionally for all FSFS filesystems.
    With non-efficient or unsupported atomics, the filesystem would print a
    warning, so handle it non-abortively.  Making assumptions about this
    test XFail-ing with both svn_named_atomic__is_efficient() and
    svn_named_atomic__is_supported() hugely relies on the internal
    implementation details, and there is nothing good with that.  So we ...
  (test_funcs): ...mark this test as WIMP instead of an XFAIL and tell the
    readers when it does fail.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1619153 13f79535-47bb-0310-9956-ffa450edef68