tests: use `tr -d` and not `tr --delete` as the latter is absent on BSD tr(1) stable

tests: use `tr -d` and not `tr --delete` as the latter is absent on BSD tr(1)

Differential Revision: https://phab.mercurial-scm.org/D6729

--- a/tests/test-merge-combination.t	Mon Aug 12 14:00:19 2019 -0400
+++ b/tests/test-merge-combination.t	Fri Aug 16 15:41:53 2019 +0300
@@ -57,7 +57,7 @@
   >                fi
   >           else expected=a
   >           fi
-  >           got=`hg log -r 3 --template '{files}\n' | tr --delete 'e '`
+  >           got=`hg log -r 3 --template '{files}\n' | tr -d 'e '`
   >           if [ "$got" = "$expected" ]
   >           then echo "$line$conflicts: agree on \"$got\""
   >           else echo "$line$conflicts: hg said \"$got\", expected \"$expected\""