When processing subunit streams we attempt to discover where the
(s)testr commands are located. For some reason the output of our scripts
that do this very occasionally emit a newline before the command path.
When this happens we were using the blank line as the command path which
fails as that empty command results in our command arguments being
interpreted as the command path.
Attempt to address this by trimming the stdout instead of taking the
first line. This should remove the leading and trailing newlines and
give us only the command itself.
Change-Id: Id651e019cf3d0b7ab37fdf9df04be249ea7f7af6
0206d6671a