Fix failing test blocking v0.20.6 release (#68)

Mockery 2.4+ deprecated `Mockery.Macro.mockable/2` based on `Mix.env/0`
and the mix.exs constraint (~> 2.1) resolved to 2.5.0, emitting compile
warnings that fail CI's `mix test --warnings-as-errors`.

Enable Mockery explicitly via `config :mockery, enable: true` in
config/test.exs and switch the converter to `use Mockery.Macro` so the
`@compile {:no_warn_undefined, Mockery.Proxy.MacroProxy}` directive is
injected, silencing the undefined-proxy warnings.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: FelonEkonom

config/config.exs

@@ -0,0 +1,5 @@
+import Config
+
+if config_env() == :test do
+  import_config "test.exs"
+end

config/test.exs

@@ -0,0 +1,3 @@
+import Config
+
+config :mockery, enable: true

lib/membrane_ffmpeg_swresample_plugin/converter.ex

@@ -5,7 +5,7 @@ defmodule Membrane.FFmpeg.SWResample.Converter do
   """
 
   use Membrane.Filter
-  import Mockery.Macro
+  use Mockery.Macro
 
   require Membrane.Logger