tests: Require base 10 for the string specifying the number of open fd's

The third arg to strtol() specifies the base to assume for the number.
When 0 is passed, as is currently done in wayland-client.c, hexadecimal
and octal numbers are permitted and automatically detected and
converted.

exec-fd-leak-checker's single argument is the count of file descriptors
it should expect to be open.  We should expect this to be specified only
as a decimal number, there's no reason why one would want to use octal
or hexadecimal for that.

Suggested by Yong Bakos.

Signed-off-by: Bryce Harrington <bryce@osg.samsung.com>
Reviewed-by: Yong Bakos <ybakos@humanoriented.com>