test(integration): Add additional custom error handler tests

[zsistla]

Added additional tests to exercise a variety of return values from a custom error handler.

[newrelic-php-agent-bot]

Test Suite	Status	Result
Multiverse	✅	9/9 passing

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.76%. Comparing base (6def121) to head (a272070).

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #869   +/-   ##
=======================================
  Coverage   78.76%   78.76%           
=======================================
  Files         193      193           
  Lines       27125    27125           
=======================================
  Hits        21365    21365           
  Misses       5760     5760           

Flag	Coverage Δ
agent-for-php-7.0	77.82% <ø> (ø)
agent-for-php-7.1	77.58% <ø> (ø)
agent-for-php-7.2	78.15% <ø> (ø)
agent-for-php-7.3	78.17% <ø> (ø)
agent-for-php-7.4	77.87% <ø> (ø)
agent-for-php-8.0	77.90% <ø> (ø)
agent-for-php-8.1	77.88% <ø> (ø)
agent-for-php-8.2	77.48% <ø> (ø)
agent-for-php-8.3	77.48% <ø> (ø)

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[lavarou]

Thanks for adding these!