chore(test): improve resource handling in tests

[igorbernstein2]

• fix resource cleanup: time based resource cleanup expected a prefix followed by a number, but some tests inserted a string in between which would prevent clean. Now all prefixes are generated in a single place
• fix cleanup contention: concurrent tests will try to cleanup resources at the same time. Mitigate this by swallowing not found error
• fix concurrent test runs: the prefix was based on time only, which would causes tests running in the same second to clobber each other
• optimize backup test to only create a single new instance

[codecov]

Codecov Report

Merging #801 (d6aab3e) into master (04f8ad4) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #801      +/-   ##
============================================
- Coverage     83.47%   83.46%   -0.02%     
  Complexity     1315     1315              
============================================
  Files           114      114              
  Lines          7806     7806              
  Branches        446      446              
============================================
- Hits           6516     6515       -1     
- Misses         1034     1036       +2     
+ Partials        256      255       -1     

Impacted Files	Coverage Δ	Complexity Δ
...om/google/cloud/bigtable/emulator/v2/Emulator.java	59.83% <0.00%> (-0.82%)	14.00% <0.00%> (ø%)

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