tests: replace `echo -n` with `printf` per check-code

Differential Revision: https://phab.mercurial-scm.org/D3749

tests/test-commit-interactive-curses.t

@@ -215,7 +215,7 b' Amend option works'
   +hello world
 
 Make file empty
-  $ echo -n > x
+  $ printf "" > x
   $ cat <<EOF >testModeCommands
   > X
   > EOF