Tests: indentation: Test the indent stack

The "indentation stack" is iteratively built by the `check()` function
of the indentation rule. It is important, since everything in the rule
relies on it.

This patch adds tests to make sure the stack is correctly built for some
known structures.

yamllint/rules/indentation.py

@@ -155,6 +155,7 @@ CONF = {'spaces': int,
         'check-multi-line-strings': bool}
 
 ROOT, B_MAP, F_MAP, B_SEQ, F_SEQ, B_ENT, KEY, VAL = range(8)
+labels = ('ROOT', 'B_MAP', 'F_MAP', 'B_SEQ', 'F_SEQ', 'B_ENT', 'KEY', 'VAL')
 
 
 class Parent(object):
@@ -165,6 +166,9 @@ class Parent(object):
         self.explicit_key = False
         self.implicit_block_seq = False
 
+    def __repr__(self):
+        return '%s:%d' % (labels[self.type], self.indent)
+
 
 def check_scalar_indentation(conf, token, context):
     if token.start_mark.line == token.end_mark.line: