Testsuite: strip trailing space at -bt output

Depending on the length of the ip address, Exim adds
spaces to -bt output, to make all lines the same length (why? Maybe to
add port= or mx= later in a nicely aligned manner)

compare:
    host 127.0.0.1     [127.0.0.1]
    host 10.0.0.1      [10.0.0.1]-

vs:
    host 127.0.0.1     [127.0.0.1]--
    host 169.16.16.16  [169.16.16.10]