Add test to validate behavior for parseProtobufFlag

```
~>  go test ./plugin/... -v
=== RUN   TestPluginServerRandom
--- PASS: TestPluginServerRandom (0.00s)
=== RUN   TestSet
--- PASS: TestSet (0.00s)
=== RUN   TestSetProtobufArgParsing
=== RUN   TestSetProtobufArgParsing/no_--protobuf_argument_provided
=== RUN   TestSetProtobufArgParsing/providing_--protobuf_as_first_argument
=== RUN   TestSetProtobufArgParsing/providing_--protobuf_as_last_argument
=== RUN   TestSetProtobufArgParsing/providing_--protobuf_as_middle_argument
--- PASS: TestSetProtobufArgParsing (0.00s)
    --- PASS: TestSetProtobufArgParsing/no_--protobuf_argument_provided (0.00s)
    --- PASS: TestSetProtobufArgParsing/providing_--protobuf_as_first_argument (0.00s)
    --- PASS: TestSetProtobufArgParsing/providing_--protobuf_as_last_argument (0.00s)
    --- PASS: TestSetProtobufArgParsing/providing_--protobuf_as_middle_argument (0.00s)
PASS
ok      github.com/hashicorp/packer-plugin-sdk/plugin   0.249s

```

plugin/set_test.go

@@ -69,3 +69,58 @@ func TestSet(t *testing.T) {
 		t.Fatalf("Unexpected error: %s", diff)
 	}
 }
+
+func TestSetProtobufArgParsing(t *testing.T) {
+	testCases := []struct {
+		name     string
+		useProto bool
+		in, out  []string
+	}{
+		{
+			name:     "no --protobuf argument provided",
+			in:       []string{"example", "example-2"},
+			out:      []string{"example", "example-2"},
+			useProto: false,
+		},
+		{
+			name:     "providing --protobuf as first argument",
+			in:       []string{"--protobuf", "example", "example-2"},
+			out:      []string{"example", "example-2"},
+			useProto: true,
+		},
+		{
+			name:     "providing --protobuf as last argument",
+			in:       []string{"example", "example-2", "--protobuf"},
+			out:      []string{"example", "example-2"},
+			useProto: true,
+		},
+		{
+			name:     "providing --protobuf as middle argument",
+			in:       []string{"example", "--protobuf", "example-2"},
+			out:      []string{"example", "example-2"},
+			useProto: true,
+		},
+		//{
+		//name:     "providing --protobuf multiple times",
+		//in:       []string{"--protobuf", "--protobuf", "example-2"},
+		//out:      []string{"example-2"},
+		//useProto: true,
+		//},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			set := NewSet()
+			got := set.parseProtobufFlag(tc.in...)
+
+			if diff := cmp.Diff(got, tc.out); diff != "" {
+				t.Errorf("Unexpected args: %s", diff)
+			}
+
+			if set.useProto != tc.useProto {
+				t.Errorf("expected useProto to be %t when %s but got %t", tc.useProto, tc.name, set.useProto)
+			}
+		})
+
+	}
+}