op-challenger: Ensure mocked claims have non-negative clocks

[Inphi]

A negative clock value prevents a recent version of upstream geth's abi package from decoding mocked claims. In Go, zero-valued time.Time values predates the year 1970, which results in a negative value when converted to a Unix timestamp.

This patch is done in anticipation of an update to the op-geth dependency that breaks this test.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.27%. Comparing base (23db42a) to head (1fc57bb).
⚠️ Report is 6 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff              @@
##           develop   #16882       +/-   ##
============================================
+ Coverage    45.55%   82.27%   +36.71%     
============================================
  Files         1454      161     -1293     
  Lines       118595     9083   -109512     
============================================
- Hits         54028     7473    -46555     
+ Misses       60776     1469    -59307     
+ Partials      3791      141     -3650     

Flag	Coverage Δ
cannon-go-tests-64	67.97% <ø> (-0.76%)	⬇️
contracts-bedrock-tests	96.25% <ø> (ø)

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see 1298 files with indirect coverage changes

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