BF(TST): Relax matching of git-annex error message about unsafe drop

What matters really is that we fail and that there is "unsafe" and some message
from git-annex.  In [10.20231129-18-gfd0b510573](https://git.kitenet.net/index.cgi/git-annex.git/commit/?id=fd0b5105731b17899d79cade82436320ab85cb56)
git-annex changed message to become better but I do not think we should overcomplicate
test ot match both messages exactly since who knows what other improvements
might come later again.

datalad/distributed/tests/test_drop.py

@@ -419,9 +419,7 @@ def test_safetynet(otherpath=None, origpath=None, clonepath=None):
 
     res = cloneds.drop(what='all', on_failure='ignore')
     assert_in_results(res, action="drop", status="error")
-    ok_(res[0]['message'].startswith(
-        "unsafe\nCould only verify the existence of "
-        "0 out of 1 necessary"),
+    ok_(res[0]['message'].startswith("unsafe\nCould"),
         msg=f"Results were {res}")
     ok_(cloneds.is_installed())