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/nl.tests

@@ -0,0 +1,39 @@
+#!/bin/sh
+# Copyright 2021 by Ron Yorston
+# Licensed under GPLv2, see file LICENSE in this source tree.
+
+. ./testing.sh
+
+# testing "test name" "commands" "expected result" "file input" "stdin"
+
+testing "nl numbers all lines" \
+	"nl -b a input" \
+"\
+     1	line 1
+     2	
+     3	line 3
+" \
+	"line 1\n\nline 3\n" \
+	""
+
+testing "nl numbers non-empty lines" \
+	"nl -b t input" \
+"\
+     1	line 1
+       
+     2	line 3
+" \
+	"line 1\n\nline 3\n" \
+	""
+
+testing "nl numbers no lines" \
+	"nl -b n input" \
+"\
+       line 1
+       
+       line 3
+" \
+	"line 1\n\nline 3\n" \
+	""
+
+exit $FAILCOUNT