coalib/abstractions: Add LinterClass.py

Introduce a virtual base class for linters and use it to
improve the check inside `LocalBearTestHelper`.
i.e. Instead of checking `hasattr(cls, 'process_output')`, an
`isinstance(cls, LinterClass)` is safer and easier to understand.

Related to coala#4594

coalib/bearlib/abstractions/Linter.py

@@ -8,6 +8,7 @@
 from subprocess import check_call, CalledProcessError, DEVNULL
 from types import MappingProxyType
 
+from coalib.bearlib.abstractions.LinterClass import LinterClass
 from coalib.bears.LocalBear import LocalBear
 from coalib.bears.GlobalBear import GlobalBear
 from coala_utils.ContextManagers import make_temp
@@ -737,6 +738,7 @@ def create_arguments(config_file):
     result_klass = type(klass.__name__, (klass, LinterBaseClass), {
         '__module__': klass.__module__})
     result_klass.__doc__ = klass.__doc__ or ''
+    LinterClass.register(result_klass)
     return result_klass
 
 
@@ -835,6 +837,11 @@ def linter(executable: str,
     and ``use_stderr=False`` raises a ``ValueError``. By default ``use_stdout``
     is ``True`` and ``use_stderr`` is ``False``.
 
+    Every ``linter`` is also a subclass of the ``LinterClass`` class.
+
+    >>> issubclass(XLintBear, LinterClass)
+    True
+
     Documentation:
     Bear description shall be provided at class level.
     If you document your additional parameters inside ``create_arguments``,

coalib/bearlib/abstractions/LinterClass.py

@@ -0,0 +1,7 @@
+from abc import ABCMeta
+
+
+class LinterClass(metaclass=ABCMeta):
+    """
+    Every ``linter`` is also a subclass of the ``LinterClass`` class.
+    """