Use PANIC_ON_INVARIANT_VIOLATED=true for integration tests

[robskillington]

What this PR does / why we need it:

This re-enables using PANIC_ON_INVARIANT_VIOLATED=true for integration tests, since the root of the issue that led us to disabling it is fixed with PR #1531.

Special notes for your reviewer:

Does this PR introduce a user-facing and/or backwards incompatible change?:

NONE

Does this PR require updating code package or user-facing documentation?:

NONE

[codecov]

Codecov Report

Merging #1622 into master will increase coverage by 9.4%.
The diff coverage is 0%.

@@           Coverage Diff            @@
##           master   #1622     +/-   ##
========================================
+ Coverage    62.5%   71.9%   +9.4%     
========================================
  Files         952     954      +2     
  Lines       79134   79171     +37     
========================================
+ Hits        49518   56996   +7478     
+ Misses      25897   18453   -7444     
- Partials     3719    3722      +3

Flag	Coverage Δ
#aggregator	82.3% <ø> (+8.1%)	⬆️
#cluster	85.7% <ø> (+15.7%)	⬆️
#collector	63.7% <ø> (ø)	⬆️
#dbnode	79.9% <0%> (+6.1%)	⬆️
#m3em	73.2% <ø> (ø)	⬆️
#m3ninx	73.9% <ø> (+8.5%)	⬆️
#m3nsch	51.1% <ø> (ø)	⬆️
#metrics	17.6% <ø> (ø)	⬆️
#msg	74.7% <ø> (ø)	⬆️
#query	67.1% <ø> (+21.9%)	⬆️
#x	87.2% <ø> (+8.2%)	⬆️

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