test: help finit know its under test

This trick can also be used by others who want to run Finit in an
unshare.  Set the container environment variable to 'unshare',
like lxc and docker do for their products.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

src/helpers.c

@@ -551,7 +551,8 @@ int in_container(void)
 	const char *containers[] = {
 		"lxc",
 		"docker",
-		"kubepod"
+		"kubepod",
+		"unshare"
 	};
 	const char *files[] = {
 		"/run/.containerenv",