tests/util/grub-shell: Trim line should always be matched from the beginning of the...

tests/util/grub-shell: Trim line should always be matched from the beginning of the line

When turning on shell tracing the trim line will be output before we
actually want to start the trim. However, in this case the trim line never
starts from the beginning of the line. So start trimming from the correct
line by matching from the beginning of the line.

Signed-off-by: Glenn Washburn <development@efficientek.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>