VHDLLintBear: Remove use of generic no cover

Add test to check for prerequisites

Closes coala#2775

bears/vhdl/VHDLLintBear.py

@@ -39,7 +39,7 @@ class VHDLLintBear:
     CAN_DETECT = {'Formatting'}
 
     @classmethod
-    def check_prerequisites(cls):  # pragma: no cover
+    def check_prerequisites(cls):
         if which('perl') is None:
             return 'perl is not installed.'
         elif which('bakalint.pl') is None:

tests/vhdl/VHDLLintBearTest.py

@@ -1,7 +1,24 @@
+from unittest import TestCase, mock
+
 from bears.vhdl.VHDLLintBear import VHDLLintBear
 from coalib.testing.LocalBearTestHelper import verify_local_bear
 
 
-VHDLLintBearTest = verify_local_bear(VHDLLintBear,
-                                     ('test',),
-                                     ('\t',))
+class VHDLLintBearPrerequisiteTest(TestCase):
+    def test_check_prerequisites(self):
+        with mock.patch('bears.vhdl.VHDLLintBear.which') as mock_which:
+            mock_which.side_effect = [None]
+            self.assertEqual(VHDLLintBear.check_prerequisites(),
+                             'perl is not installed.')
+
+            mock_which.side_effect = ['path/to/perl', None]
+            self.assertEqual(VHDLLintBear.check_prerequisites(),
+                             'bakalint is missing. Download it from '
+                             '<http://fpgalibre.sourceforge.net/'
+                             'ingles.html#tp46> and put it into your PATH.')
+
+            mock_which.side_effect = ['path/to/perl', 'path/to/bakalint']
+            self.assertEqual(VHDLLintBear.check_prerequisites(), True)
+
+
+VHDLLintBearTest = verify_local_bear(VHDLLintBear, ('test',), ('\t',))