tests: skip video file tests if ffmpeg isn't installed.

app/logical/media_file.rb

@@ -45,6 +45,10 @@ class MediaFile
     end
   end
 
+  def self.videos_enabled?
+    system("ffmpeg -version > /dev/null") && system("mkvmerge --version > /dev/null")
+  end
+
   def initialize(file, **options)
     @file = file
   end

app/logical/media_file/ugoira.rb

@@ -2,10 +2,6 @@ class MediaFile::Ugoira < MediaFile
   class Error < StandardError; end
   attr_reader :frame_data
 
-  def self.conversion_enabled?
-    system("ffmpeg -version > /dev/null") && system("mkvmerge --version > /dev/null")
-  end
-
   def initialize(file, frame_data: {}, **options)
     super(file, **options)
     @frame_data = frame_data
@@ -31,7 +27,7 @@ class MediaFile::Ugoira < MediaFile
 
   # XXX should take width and height and resize image
   def convert
-    raise NotImplementedError, "can't convert ugoira to webm: ffmpeg or mkvmerge not installed" unless self.class.conversion_enabled?
+    raise NotImplementedError, "can't convert ugoira to webm: ffmpeg or mkvmerge not installed" unless self.class.videos_enabled?
 
     Dir.mktmpdir("ugoira-#{md5}") do |tmpdir|
       output_file = Tempfile.new(["ugoira-conversion", ".webm"], binmode: true)

app/logical/media_file/video.rb

@@ -22,6 +22,8 @@ class MediaFile::Video < MediaFile
   private
 
   def video
+    raise NotImplementedError, "can't process videos: ffmpeg or mkvmerge not installed" unless self.class.videos_enabled?
+
     FFMPEG::Movie.new(file.path)
   end