Add -coverpkg option when running unit test to calculate coverage across packages (#9913)

(cherry picked from commit 7c3157b)

Author: pchila

dev-tools/mage/gotest.go

@@ -256,6 +256,7 @@ func GoTest(ctx context.Context, params GoTestArgs) error {
 		testArgs = append(testArgs,
 			"-covermode=atomic",
 			"-coverprofile="+params.CoverageProfileFile,
+			"-coverpkg=./...", // needed to calculate the coverage across the elastic-agent packages
 		)
 	}