Bigtable: Update integration tests to use the bundled emulator

[igorbernstein2]

Also update the emulator bundler to download artifacts in parallel

Fixes #<issue_number_goes_here> (it's a good idea to open an issue first for context and/or discussion)

[codecov]

Codecov Report

Merging #5769 into master will increase coverage by 0.67%.
The diff coverage is n/a.

@@ Coverage Diff @@## master #5769 +/- ##
============================================
+ Coverage 46.5% 47.17% +0.67% - Complexity 23679 25115 +1436 
============================================
Files 2389 2389 Lines 254581 259836 +5255 Branches 28482 29422 +940 ============================================
+ Hits 118385 122586 +4201 - Misses 127412 128320 +908 - Partials 8784 8930 +146

Impacted Files	Coverage Δ	Complexity Δ
...re/src/main/java/com/google/cloud/BaseService.java	55.55% <0%> (-6.95%)	2% <0%> (ø)
...re/src/main/java/com/google/cloud/RetryHelper.java	50% <0%> (-3.85%)	2% <0%> (ø)
...le/cloud/compute/deprecated/DeprecationStatus.java	88.63% <0%> (-3.13%)	20% <0%> (ø)
...m/google/cloud/tasks/v2beta3/CloudTasksClient.java	57.43% <0%> (-2.68%)	67% <0%> (+16%)
...va/com/google/cloud/tasks/v2/CloudTasksClient.java	57.43% <0%> (-2.68%)	67% <0%> (+16%)
...m/google/cloud/tasks/v2beta2/CloudTasksClient.java	57.14% <0%> (-2.39%)	79% <0%> (+19%)
...oogle/cloud/firestore/v1/FirestoreAdminClient.java	59.84% <0%> (-2.23%)	38% <0%> (+8%)
.../cloud/scheduler/v1beta1/CloudSchedulerClient.java	55% <0%> (-2.15%)	35% <0%> (+8%)
...oogle/cloud/scheduler/v1/CloudSchedulerClient.java	55% <0%> (-2.15%)	35% <0%> (+8%)
...gle/cloud/bigtable/data/v2/BigtableDataClient.java	89.58% <0%> (-1.91%)	32% <0%> (ø)
... and 646 more

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[kolea2]

Should we add these instructions to the emulator package?

[igorbernstein2]

No, these instructions are for running the tests. The emulator package has instructions focusing on how a customer can use the emulator for their own tests:

https://github.com/googleapis/google-cloud-java/tree/master/google-cloud-testing/google-cloud-bigtable-emulator

[igorbernstein2]

Thanks for reviewing!