testcases/lib: Add tst_sleep

So that we can finally sleep for less than 1 second in shell testcases.

+ documentation in the test-writing-guidelines

Signed-off-by: Cyril Hrubis <chrubis@suse.cz>
diff --git a/testcases/lib/.gitignore b/testcases/lib/.gitignore
new file mode 100644
index 0000000..8886b34
--- /dev/null
+++ b/testcases/lib/.gitignore
@@ -0,0 +1 @@
+tst_sleep