Import Export test fix

[aasthabharill]

b/531449982

Reason for using default partition instead of creating a new one:

• When a spanner instance is created, Spanner implicitly provisions a built-in partition named "default" that shares the instance's region configuration.
• Spanner strictly prohibits creating a new custom instance partition that shares the exact same region as another partition - so we can't use the spanner instance region due to the default partition that was created
• Therefore, to successfully create a custom partition, the base instance must be created in one configuration (e.g., nam3), and the custom partition must be created in a different configuration (e.g., nam6).
• But across our three environments (Prod, Pre-Prod, and Cloud-devel), there isn't a universally shared set of two distinct multi-region configurations.
• So we will utilize the default partition instead of provisioning a secondary partition.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 55.70%. Comparing base (c8baa96) to head (4b03b0b).

Additional details and impacted files

@@              Coverage Diff              @@
##               main    #3986       +/-   ##
=============================================
+ Coverage     33.95%   55.70%   +21.74%     
- Complexity      503     6773     +6270     
=============================================
  Files           215     1125      +910     
  Lines         13131    68425    +55294     
  Branches       1299     7717     +6418     
=============================================
+ Hits           4459    38117    +33658     
- Misses         8308    27835    +19527     
- Partials        364     2473     +2109     

Components	Coverage Δ
spanner-templates	87.67% <ø> (∅)
spanner-import-export	68.70% <ø> (∅)
spanner-live-forward-migration	89.29% <ø> (∅)
spanner-live-reverse-replication	83.45% <ø> (∅)
spanner-bulk-migration	92.49% <ø> (∅)
gcs-spanner-dv	88.53% <ø> (∅)
see 935 files with indirect coverage changes

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