test: add a basic 'test-check-pylint.t'

We add a minimal check using pylint for one case we knows we care about:
"mutable default" argument.

We'll likely extend this over time to cover other useful checks but this is a
good starting point.

tests/test-check-pylint.t

@@ -0,0 +1,15 b''
+#require test-repo pylint hg10
+
+Run pylint for known rules we care about.
+-----------------------------------------
+
+There should be no recorded failures; fix the codebase before introducing a
+new check.
+
+Current checks:
+- W0102: no mutable default argument
+
+  $ touch $TESTTMP/fakerc
+  $ pylint --rcfile=$TESTTMP/fakerc --disable=all \
+  >   --enable=W0102 --reports=no \
+  >   mercurial hgext hgext3rd

tests/hghave.py

@@ -413,6 +413,12 b' def has_pyflakes():'
                        br"<stdin>:1: 're' imported but unused",
                        True)
 
+@check("pylint", "Pylint python linter")
+def has_pylint():
+    return matchoutput("pylint --help",
+                       br"Usage:  pylint",
+                       True)
+
 @check("pygments", "Pygments source highlighting library")
 def has_pygments():
     try: