lib: Add new tst_kvcmp for shell

Add tst_kvcmp inspired by the shell test syntax.

Now we can write the kernel version checks as:

if tst_kvcmp -ge 3.8.0 -a -le 4.3.2; then
	tst_brk TCONF "Needs kernel older than 3.8.0 or newer than 4.3.2"
fi

Signed-off-by: Cyril Hrubis <chrubis@suse.cz>
diff --git a/testcases/lib/.gitignore b/testcases/lib/.gitignore
index 0a28d61..77acb57 100644
--- a/testcases/lib/.gitignore
+++ b/testcases/lib/.gitignore
@@ -2,3 +2,4 @@
 tst_random
 tst_checkpoint
 tst_rod
+tst_kvcmp