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
$ hg init
$ echo This is file a1 > a
$ hg add a
$ hg commit -m "commit #0"
$ ls
a
$ echo This is file b1 > b
$ hg add b
$ hg commit -m "commit #1"
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
B should disappear
$ ls
a