Bigtable: Refactor TableAdmin tests to run against the emulator

[igorbernstein2]

• Share environment selector between admin & data
• Stop data clean up and rely on garbage collection instead
• extract table admin table clean up to a teardown function
• remove cleanup responsibilities from env impls

To make reviewing easier, I would recommend to "Hide whitespace changes" in the diff

[codecov]

Codecov Report

Merging #5795 into master will decrease coverage by 1.31%.
The diff coverage is n/a.

@@ Coverage Diff @@## master #5795 +/- ##
============================================
- Coverage 47.38% 46.07% -1.32% + Complexity 27182 24189 -2993 
============================================
Files 2523 2456 -67 Lines 274600 262222 -12378 Branches 31383 30362 -1021 ============================================
- Hits 130125 120823 -9302 + Misses 134862 132287 -2575 + Partials 9613 9112 -501

Impacted Files	Coverage Δ	Complexity Δ
.../java/com/google/cloud/testing/CommandWrapper.java	87.87% <0%> (-9.1%)	13% <0%> (ø)
.../cloud/datastore/testing/LocalDatastoreHelper.java	80.59% <0%> (-4.48%)	17% <0%> (ø)
...in/java/com/google/cloud/storage/StorageBatch.java	88% <0%> (-4%)	13% <0%> (ø)
...n/java/com/google/cloud/firestore/WriteResult.java	46.15% <0%> (-3.85%)	4% <0%> (ø)
...m/google/cloud/vision/v1/ImageAnnotatorClient.java	58.69% <0%> (-3.31%)	15% <0%> (-4%)
...e/cloud/vision/v1p4beta1/ImageAnnotatorClient.java	58.69% <0%> (-3.31%)	15% <0%> (-4%)
...gle/cloud/storage/testing/RemoteStorageHelper.java	61.98% <0%> (-3.06%)	9% <0%> (ø)
...s/src/main/java/com/google/cloud/dns/DnsBatch.java	87.87% <0%> (-3.04%)	20% <0%> (ø)
...ud/bigtable/data/v2/stub/BigtableStubSettings.java	81.61% <0%> (-2.96%)	14% <0%> (ø)
...e/cloud/vision/v1p3beta1/ImageAnnotatorClient.java	47.05% <0%> (-2.95%)	9% <0%> (-2%)
... and 900 more

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