selftests/nolibc: Use length of 'expected' string to check snprintf() output

selftests/nolibc: Use length of 'expected' string to check snprintf() output

Instead of requiring the test cases specifying both the length and
expected output, take the length from the expected output.
Tests that expect the output be truncated are changed to specify
the un-truncated output.

Change the strncmp() to a memcmp() with an extra check that the
output is actually terminated.

Append a '+' to the printed output (after the final ") when the output
is truncated.

Signed-off-by: David Laight <david.laight.linux@gmail.com>
Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://patch.msgid.link/20260302101815.3043-6-david.laight.linux@gmail.com
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>