[tests] Use a constant-length timestamp redaction for test determinism (#6511)

Changes our "timestamp redaction" logic to ignore the original length of
the input timestamp.

Timestamps are not always constant-width -- depending on the amount of
trailing zeros, they can have variable stringified lengths.
Unfortunately, this variability is at-odds with our goal of
deterministic test output.

In this PR, we favor the deterministic output, to avoid test flaking:
Variable space output logic is removed in favor of a stable redaction
marker. Additionally, tests with different timestamp lengths are added
to act as a regression against this particular flake.

Fixes #6509

Author: smklein