test: Improve code coverage for Acl

[irvifa]

In Acl there's still untested class particularly the IamMember. Also we can still improve the coverage by improving test for Group.

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Fixes #489 ☕️

[codecov]

Codecov Report

Merging #490 into master will increase coverage by 0.12%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #490      +/-   ##
============================================
+ Coverage     81.36%   81.48%   +0.12%     
  Complexity     1225     1225              
============================================
  Files            77       77              
  Lines          6218     6218              
  Branches        691      691              
============================================
+ Hits           5059     5067       +8     
+ Misses          802      792      -10     
- Partials        357      359       +2     

Impacted Files	Coverage Δ	Complexity Δ
...y/src/main/java/com/google/cloud/bigquery/Acl.java	71.42% <0.00%> (+6.01%)	13.00% <0.00%> (ø%)

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[irvifa]

Somehow got flaky test on:

com.google.common.truth.AssertionErrorWithFacts:
expected to contain:
    Empty column successfully added to table
but was:
    Empty column was not added.
    com.google.cloud.bigquery.BigQueryException: Provided Schema does not match Table java-docs-samples-testing:bigquery_test_dataset.test_table. Field new_896e1c11_8ba3_4037_a2de_f6d76ea69e53 is missing in new schema

	at com.example.bigquery.AddEmptyColumnIT.addEmptyColumn(AddEmptyColumnIT.java:65)

https://source.cloud.google.com/results/invocations/9eac7d0a-69ed-4040-beb9-40f796ec7d83/targets;collapsed=/github%2Fjava-bigquery%2Fsamples%2Finstall-without-bom%2Ftarget%2Ffailsafe-reports/tests