nl: ensure '-b n' option displays file content

The command 'nl -b n' should output no line numbers, just some
spaces as a placeholder followed by the actual file content.

Add tests for line numbering by cat and nl.  The correct results
were obtained from coreutils.

function                                             old     new   delta
print_numbered_lines                                 152     157      +5
.rodata                                           182456  182453      -3
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 1/1 up/down: 5/-3)                Total: 2 bytes

Signed-off-by: Ron Yorston <rmy@pobox.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

testsuite/cat.tests

@@ -22,4 +22,28 @@ testing 'cat -v' \
 	'foo\n'
 SKIP=
 
+optional FEATURE_CATN
+testing 'cat -n' \
+	'cat -n' \
+"\
+     1	line 1
+     2	
+     3	line 3
+" \
+	'' \
+	'line 1\n\nline 3\n'
+SKIP=
+
+optional FEATURE_CATN
+testing 'cat -b' \
+	'cat -b' \
+"\
+     1	line 1
+
+     2	line 3
+" \
+	'' \
+	'line 1\n\nline 3\n'
+SKIP=
+
 exit $FAILCOUNT