tests: use inline Python for revlog test

`hg debugdata` will soon stop accepting the raw path to a revlog file.
Adjust a test accordingly.

The changed test is for a security bug. So this should be reviewed
with scrutiny.

Differential Revision: https://phab.mercurial-scm.org/D4356

tests/test-revlog.t

@@ -43,5 +43,10 b' Test for CVE-2016-3630'
      rev linkrev nodeid       p1           p2
        0       2 99e0332bd498 000000000000 000000000000
        1       3 6674f57a23d8 99e0332bd498 000000000000
-  $ hg debugdata a.i 1 2>&1 | egrep 'Error:.*decoded'
-  (mercurial\.\w+\.mpatch\.)?mpatchError: patch cannot be decoded (re)
+
+  >>> from mercurial import revlog, vfs
+  >>> tvfs = vfs.vfs(b'.')
+  >>> tvfs.options = {b'revlogv1': True}
+  >>> rl = revlog.revlog(tvfs, b'a.i')
+  >>> rl.revision(1)
+  mpatchError('patch cannot be decoded',)