fix: use relative URLs for test submodules

Co-authored-by: klutchell <20458272+klutchell@users.noreply.github.com>

Author: Copilot

.gitmodules

@@ -1,9 +1,9 @@
 [submodule "tests/leviathan"]
 	path = tests/leviathan
-	url = https://github.com/balena-os/leviathan.git
+	url = ../leviathan.git
 [submodule "tests/suites/os/tests/secureboot/kernel-module-build"]
 	path = tests/suites/os/tests/secureboot/kernel-module-build
-	url = https://github.com/balena-os/kernel-module-build
+	url = ../kernel-module-build.git
 [submodule "tests/suites/os/tests/extra-firmware/kernel-module-build"]
 	path = tests/suites/os/tests/extra-firmware/kernel-module-build
-	url = https://github.com/balena-os/kernel-module-build
+	url = ../kernel-module-build.git