Fix flaky test TestJSONFormatProgress (#21124)

In TestJSONFormatProgress, the progress string was used for comparison.
However, the progress string (progress.String()) uses time.Now().UTC()
to generate the timeLeftBox which is not a fixed value and cannot be
compared reliably.

This PR fixes the issue by stripping the timeLeftBox field before doing
the comparison.

This PR fixes #21124.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>