test(bigtable): abbreviate instance and cluster names to < 32 chars

[crwilcox]

test(bigtable): abbreviate instance and cluster names to much less than 32 chars to avoid overrun

Fixes #4891

[crwilcox]

@codyoss can you create an instance

# Bigtable integration tests expect an existing instance and cluster
#  ❯ cbt createinstance gc-bt-it-instance gc-bt-it-instance \
#    gc-bt-it-cluster us-west1-b 1 SSD

[codyoss]

@crwilcox done.