fix: Remove bigtable instances left over when system tests run (#1265)

* Use the prefix from the common file for deleting

Instances are not currently being cleaned up by the reap instances function because a filter is not selecting instances based on the right prefix. It looks like this fix should solve the issue.

* fix: Use the prefix from the common file for deleting

Instances are not currently being cleaned up by the reap instances function because a filter is not selecting instances based on the right prefix. It looks like this fix should solve the issue.

system-test/bigtable.ts

@@ -25,9 +25,7 @@ import {Family} from '../src/family.js';
 import {Row} from '../src/row.js';
 import {Table} from '../src/table.js';
 import {RawFilter} from '../src/filter';
-import {generateId} from './common';
-
-const PREFIX = 'gcloud-tests-';
+import {generateId, PREFIX} from './common';
 
 describe('Bigtable', () => {
   const bigtable = new Bigtable();