[Test Failure] testIsoStringsInterpolateTz is failing on Logstash 8.x

[robbavey]

https://app.travis-ci.com/github/logstash-plugins/logstash-filter-date/builds/244341288

logstash_1_7632f9cff24c | <===========--> 85% EXECUTING [14s]> :test > 1592 tests completed> :test > Executing test org.logstash.filters.DateFilterTest<===========--> 85% EXECUTING [15s]<===========--> 85% EXECUTING [16s]> :test > 1598 tests completedlogstash_1_7632f9cff24c | org.logstash.filters.DateFilterTest > testIsoStringsInterpolateTz FAILED
logstash_1_7632f9cff24c |     java.lang.AssertionError at DateFilterTest.java:212
logstash_1_7632f9cff24c | 
logstash_1_7632f9cff24c | 1602 tests completed, 1 failed
logstash_1_7632f9cff24c | 
logstash_1_7632f9cff24c | 
logstash_1_7632f9cff24c | 
logstash_1_7632f9cff24c | <===========--> 85% EXECUTING [16s]> :test > 1602 tests completed, 1 failed<===========--> 85% EXECUTING [17s]> :testlogstash_1_7632f9cff24c | > Task :test FAILED
logstash_1_7632f9cff24c | 
logstash_1_7632f9cff24c | FAILURE: Build failed with an exception.
logstash_1_7632f9cff24c | 
logstash_1_7632f9cff24c | * What went wrong:
logstash_1_7632f9cff24c | Execution failed for task ':test'.
logstash_1_7632f9cff24c | > There were failing tests. See the report at: file:///usr/share/plugins/plugin/build/reports/tests/test/index.html
logstash_1_7632f9cff24c | 
logstash_1_7632f9cff24c | * Try:
logstash_1_7632f9cff24c | Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.
logstash_1_7632f9cff24c | 
logstash_1_7632f9cff24c | * Get more help at https://help.gradle.org
logstash_1_7632f9cff24c | 
logstash_1_7632f9cff24c | BUILD FAILED in 19s
logstash_1_7632f9cff24c | 4 actionable tasks: 2 executed, 2 up-to-date
logstash_1_7632f9cff24c | 
logstash_1_7632f9cff24c | 
logstash_1_7632f9cff24c | 
logstash_1_7632f9cff24c | <=============> 100% EXECUTING [17s]<-------------> 0% WAITINGci_logstash_1_7632f9cff24c exited with