tests: avoid FP of ls/stat-free-color.sh with newer glibc

tests: avoid FP of ls/stat-free-color.sh with newer glibc

Since glibc-2.22, specifically commit [0], the opendir() implementation
implicitly makes an additional stat call thus leading to a FP.
Seen on openSUSE:Tumbleweed since snapshot 20150821.

[0]
https://sourceware.org/git/?p=glibc.git;a=commitdiff;h=46f894d8c60a

* tests/ls/stat-free-color.sh: Change the test to verify that ls(1)
needs the same number of stat-like calls for a single, empty directory
argument as for one with a few directory entries (sub-directory,
regular file, symlink, etc.).