test(firestore): Fix TestIntegration_BulkWriter_Create

[bhshkh]

Issue: TestIntegration_BulkWriter_Create fails with error:

=== RUN   TestIntegration_BulkWriter_Create
    integration_test.go:2362: "Successful": Mismatch in error got: AlreadyExists, want: "OK"
--- FAIL: TestIntegration_BulkWriter_Create (18.36s)

Cause:
There are 2 test cases in TestIntegration_BulkWriter_Create

Before running the testcases, a Firestore document is created with ID bw_create_(current unix time).
Then, first testcase is executed which creates a document with ID bw_create_(current unix time) and expects the creation to be successful but since there is very little time lapse between the 2 ID creations, they are identical. Thus, the second document creation fails with error AlreadyExists.

Fix:
Use IDs bw_create_1_(current unix time) and bw_create_2_(current unix time)