JavaPMDBear: Remove use of generic no cover

Add test to check for prerequisites

Closes coala#2769

bears/java/JavaPMDBear.py

@@ -24,7 +24,7 @@ class JavaPMDBear:
                   'Duplication'}
 
     @classmethod
-    def check_prerequisites(cls):  # pragma: no cover
+    def check_prerequisites(cls):
         if which('bash') is None:
             return 'bash is not installed.'
         elif which('pmd') is None and which('run.sh') is None:

tests/java/JavaPMDBearTest.py

@@ -1,3 +1,5 @@
+from unittest import mock, TestCase
+
 from bears.java.JavaPMDBear import JavaPMDBear
 from coalib.testing.LocalBearTestHelper import verify_local_bear
 
@@ -29,6 +31,24 @@ class Hello {
 """
 
 
+class JavaPMDBearPrerequisiteTest(TestCase):
+    def test_check_prerequisites(self):
+        with mock.patch('bears.java.JavaPMDBear.which') as mock_which:
+            mock_which.side_effect = [None, None, None]
+            self.assertEqual(JavaPMDBear.check_prerequisites(),
+                             'bash is not installed.')
+
+            mock_which.side_effect = ['path/to/bash', None, None]
+            self.assertEqual(JavaPMDBear.check_prerequisites(),
+                             ('PMD is missing. Make sure to install it from '
+                              '<https://pmd.github.io/>'))
+
+            mock_which.side_effect = ['path/to/bash',
+                                      'path/to/pmd',
+                                      'path/to/run']
+            self.assertEqual(JavaPMDBear.check_prerequisites(), True)
+
+
 JavaPMDBearTest = verify_local_bear(
     JavaPMDBear, valid_files=(good_file,), invalid_files=(bad_file,),
     tempfile_kwargs={'suffix': '.java'})