gzip: add test that checks that -9 compresses better than -1

Signed-off-by: Natanael Copa <ncopa@alpinelinux.org>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/testsuite/gzip/gzip-compression-levels b/testsuite/gzip/gzip-compression-levels
new file mode 100644
index 0000000..6d9a13d
--- /dev/null
+++ b/testsuite/gzip/gzip-compression-levels
@@ -0,0 +1,5 @@
+# FEATURE: CONFIG_FEATURE_GZIP_LEVELS
+
+level1=$(busybox gzip -c -1 $(which busybox) | wc -c)
+level9=$(busybox gzip -c -9 $(which busybox) | wc -c)
+test $level1 -gt $level9