tests/filter/stderr.expected

   1 subdirmk: warning (unknown-warning): ./Prefix.sd.mk:2: unknown warning many-requests-for-unknown-warning requested
   2 subdirmk: warning (single-char-var): ./Dir.sd.mk:8: possibly confusing unbracketed single-char $-expansion
   3 subdirmk: warning (local+global): saw both WARN and &WARN
   4  saw WARN at ./Dir.sd.mk:6
   5  saw WARN at ./Dir.sd.mk:9
   6  saw &WARN at ./Dir.sd.mk:7
   7  saw &WARN at ./Dir.sd.mk:10
   8  saw &WARN at ./Dir.sd.mk:11
   9 subdirmk: warning (unknown-warning): ./sub/dir/Dir.sd.mk:3: attempt to suppress unknown warning(s) `some-unknown-warning'