tests: add missing b'' prefix in test-check-code.t
It now passes on Python 3.
# skip-blame b prefix
Differential Revision: https://phab.mercurial-scm.org/D4250
https://bz.mercurial-scm.org/1089
$ hg init
$ mkdir a
$ echo a > a/b
$ hg ci -Am m
adding a/b
$ hg rm a
removing a/b
$ hg ci -m m a
$ mkdir a b
$ echo a > a/b
$ hg ci -Am m
adding a/b
$ hg rm a
removing a/b
$ cd b
Relative delete:
$ hg ci -m m ../a
$ cd ..