Reference correct branch name from m2e-core-tests.git (#2083)

It has been renamed from master to main

Author: kwin

.gitmodules

@@ -1,4 +1,4 @@
 [submodule "m2e-core-tests"]
 	path = m2e-core-tests
 	url = https://github.com/eclipse-m2e/m2e-core-tests.git
-	branch = master
+	branch = main