backend: let ParseConfig return a Config pointer

In order to change the backend initialization in `global.go` to be able
to generically call cfg.ApplyEnvironment() for supported backends, the
`interface{}` returned by `ParseConfig` must contain a pointer to the
configuration.

An alternative would be to use reflection to convert the type from
`interface{}(Config)` to `interface{}(*Config)` (from value to pointer
type). However, this would just complicate the type mess further.

internal/backend/local/config_test.go

@@ -0,0 +1,18 @@
+package local
+
+import (
+	"testing"
+
+	"github.com/restic/restic/internal/backend/test"
+)
+
+var configTests = []test.ConfigTestData[Config]{
+	{S: "local:/some/path", Cfg: Config{
+		Path:        "/some/path",
+		Connections: 2,
+	}},
+}
+
+func TestParseConfig(t *testing.T) {
+	test.ParseConfigTester(t, ParseConfig, configTests)
+}