Unescaped filename in regex pattern causes test failure on Windows

<pre>t\12-no-newline.t .........
#   Failed test 'test the appearance of a normal warning'
t\12-no-newline.t ......... 1/? #   at t\12-no-newline.t line 11.
#                   'a normal warning at t\12-no-newline.t line 10.
# '
#     doesn't match '(?^:^a normal warning at t\12-no-newline.t line 10\.?\n$)'

#   Failed test 'warning has origin properly added when it was lacking'
#   at t\12-no-newline.t line 23.
#                   'a warning with no newline at t\12-no-newline.t line 22.
# '
#     doesn't match '(?^:^a warning with no newline at t\12-no-newline.t line 22.?\n$)'
# Looks like you failed 2 tests of 2.</pre>