tests: avoid test failure with non-ancient autoconf

Running "make check" would fail with this:
  T-nonl.at:30: error: m4_divert_push: cannot change diversion
  to `GROW' inside m4_expand
* tests/T-nonl.at: Use printf in place of AS_ECHO_N.
These days, printf should work for everyone.

tests/T-nonl.at

@@ -29,9 +29,8 @@ AT_KEYWORDS([files-from nonewline nonl T-nonl])
 
 AT_TAR_CHECK([
 genfile --length=0 --file empty
-AS_ECHO_N(c) > 1.nonl
-echo d > 2.nonl
-AS_ECHO_N(e) >> 2.nonl
+printf c > 1.nonl
+printf 'd\ne' > 2.nonl
 touch a b c d e
 AT_DATA([filelist],[a
 b
@@ -59,4 +58,3 @@ c
 [],[],[],[ustar])
 
 AT_CLEANUP
-