add deadletter to PubsubAvroToBigQueryIT

[Polber]

PubsubAvroToBigQueryIT now includes deadletter messages to ensure DLQ is populated correctly.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 41.23%. Comparing base (f1a52d0) to head (becb04b). Report is 147 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1472      +/-   ##
============================================
- Coverage     45.53%   41.23%   -4.30%     
- Complexity      717     2928    +2211     
============================================
  Files           301      763     +462     
  Lines         16171    44393   +28222     
  Branches       1609     4754    +3145     
============================================
+ Hits           7363    18307   +10944     
- Misses         8266    24542   +16276     
- Partials        542     1544    +1002     

Components	Coverage Δ
spanner-templates	61.37% <ø> (+2.31%)	:arrow_up:
spanner-import-export	64.42% <ø> (∅)
spanner-live-forward-migration	73.88% <ø> (ø)
spanner-live-reverse-replication	49.66% <ø> (ø)
spanner-bulk-migration	82.07% <ø> (ø)

see 480 files with indirect coverage changes