xfstests: Change the diff output of failed tests

xfstests: Change the diff output of failed tests

Currently the entire diff is printed to the stdout in the case that the
test output template differs from the actual test run. However in some
cases the diff can be _very_ long. This commit changes it so that we
print only first 10 lines of the diff.

Also indent the diff output from the left by four spaces for better
output readability.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Reviewed-by: Ben Myers <bpm@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>