tests: show unusual error message for `hg log --style coal`

It turns out that we show the full path when the given style name
matches a subdirectory of `mercurial/templates/`.

Differential Revision: https://phab.mercurial-scm.org/D8803

tests/test-template-map.t

@@ -1281,6 +1281,12 b' Error if style is a directory:'
   abort: Is a directory: 'somedir'
   [255]
 
+Error if style is a directory whose name is a built-in style:
+
+  $ hg log --style coal
+  abort: Is a directory: '*/mercurial/templates/coal' (glob)
+  [255]
+
 Error if style missing key:
 
   $ echo 'q = q' > t