fix: delete old instances then create new instance

[skuruppu]

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Fixes #954 🦕

[skuruppu]

Totally not tested yet, let's see if this runs.

[hengfengli]

I don't think this would work. INSTANCE_ID is generated by date and I guess yesterday's instance was not deleted. All the deleting instances calls use the instance ID based on today not yesterday or past days.

const INSTANCE_ID = process.env.SPANNERTEST_INSTANCE || `test-instance-${date}`;
const INSTANCE_ALREADY_EXISTS = !!process.env.SPANNERTEST_INSTANCE;

[skuruppu]

It worked! Except for the fact that a backup couldn't be completed within the test timeout period :(

[hengfengli]

I see. It deletes all outdated instances first. It makes sense. I misunderstood how the deletion of instances work. I thought it only delete the current testing instance. LGTM.

[codecov]

Codecov Report

Merging #955 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #955   +/-   ##
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  Coverage   98.20%   98.20%           
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  Files          21       21           
  Lines       19959    19959           
  Branches     1069     1069           
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  Hits        19601    19601           
  Misses        355      355           
  Partials        3        3           

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