better tolerance in RG tests

The first fix avoids contaminating all future tests if the plugin fails to
load. The second skips the CLI backend tests when the appropriate tool is not
available (just as we do with the GStreamer tests).

test/test_replaygain.py

@@ -16,6 +16,7 @@
 import os
 import shutil
 from glob import glob
+import subprocess
 
 import _common
 from _common import unittest
@@ -36,7 +37,14 @@ class ReplayGainCliTestBase(TestHelper):
 
     def setUp(self):
         self.setup_beets()
-        self.load_plugins('replaygain')
+
+        try:
+            self.load_plugins('replaygain')
+        except:
+            self.teardown_beets()
+            self.unload_plugins()
+            raise
+
         self.config['replaygain']['backend'] = self.backend
         self.config['plugins'] = ['replaygain']
         self.setupLibrary(2)
@@ -126,6 +134,20 @@ class ReplayGainGstCliTest(ReplayGainCliTestBase, unittest.TestCase):
 class ReplayGainCmdCliTest(ReplayGainCliTestBase, unittest.TestCase):
     backend = u'command'
 
+    def setUp(self):
+        # Check for the backend command.
+        for command in ['mp3gain', 'aacgain']:
+            try:
+                subprocess.check_call([command, '-v'])
+            except OSError:
+                pass
+            else:
+                break
+        else:
+            self.skipTest('no *gain command found')
+
+        super(ReplayGainCmdCliTest, self).setUp()
+
 
 def suite():
     return unittest.TestLoader().loadTestsFromName(__name__)