Drop previous external tables prior to running tests

### Describe the bug
On a recent PR #148 - there was a bug but the CI tests was passing because even if there's an error with our `dbt run-operation ...` commands, there still exist the same table that was created in previous CI tests - this causes the final `dbt test` command to pass and thus the CI test as a whole is passing.

https://github.com/dbt-labs/dbt-external-tables/blob/e38cc50310de1a739809a2b48f7fa2790699e857/run_test.sh#L37-L40

### Steps to reproduce

### Expected results

### Actual results

### Screenshots and log output
https://app.circleci.com/pipelines/github/dbt-labs/dbt-external-tables/229/workflows/7fc76d6f-28eb-48fc-ba82-77f77ba33a97/jobs/1052

```
14:30:55 Running with dbt=1.2.0-rc1
14:30:55 No prep necessary, skipping
14:30:58 Running with dbt=1.2.0-rc1
14:30:59 Unable to do partial parsing because config vars, config profile, or config target have changed
14:31:00 1 of 4 START external source dbt_external_tables_integration_tests_bigquery.people_csv_unpartitioned
14:31:01 1 of 4 (1) create or replace external table `************`.`dbt_external_tables_integration...
14:31:02 Encountered an error while running operation: Database Error
Syntax error: Unexpected "<" at [6:17]
14:31:05 Running with dbt=1.2.0-rc1
14:31:05 Unable to do partial parsing because config vars, config profile, or config target have changed
14:31:07 1 of 4 START external source dbt_external_tables_integration_tests_bigquery.people_csv_unpartitioned
14:31:07 1 of 4 SKIP
14:31:07 2 of 4 START external source dbt_external_tables_integration_tests_bigquery.people_csv_partitioned
14:31:08 2 of 4 SKIP
14:31:08 3 of 4 START external source dbt_external_tables_integration_tests_bigquery.people_csv_schema_auto_detect
14:31:08 3 of 4 SKIP
14:31:08 4 of 4 START external source dbt_external_tables_integration_tests_bigquery.people_csv_override_uris
14:31:08 4 of 4 SKIP
14:31:11 Running with dbt=1.2.0-rc1
14:31:11 Found 0 models, 4 tests, 0 snapshots, 0 analyses, 519 macros, 0 operations, 1 seed file, 5 sources, 0 exposures, 0 metrics
14:31:11
14:31:12 Concurrency: 1 threads (target='bigquery')
14:31:12
14:31:12 1 of 4 START test dbt_utils_source_equality_bigquery_external_people_csv_override_uris_id__first_name__last_name__email__ref_people_ [RUN]
14:31:16 1 of 4 PASS dbt_utils_source_equality_bigquery_external_people_csv_override_uris_id__first_name__last_name__email__ref_people_ [PASS in 4.42s]
14:31:16 2 of 4 START test dbt_utils_source_equality_bigquery_external_people_csv_partitioned_id__first_name__last_name__email__ref_people_ [RUN]
14:31:18 2 of 4 PASS dbt_utils_source_equality_bigquery_external_people_csv_partitioned_id__first_name__last_name__email__ref_people_ [PASS in 1.75s]
14:31:18 3 of 4 START test dbt_utils_source_equality_bigquery_external_people_csv_schema_auto_detect_id__first_name__last_name__email__ref_people_ [RUN]
14:31:20 3 of 4 PASS dbt_utils_source_equality_bigquery_external_people_csv_schema_auto_detect_id__first_name__last_name__email__ref_people_ [PASS in 2.08s]
14:31:20 4 of 4 START test dbt_utils_source_equality_bigquery_external_people_csv_unpartitioned_id__first_name__last_name__email__ref_people_ [RUN]
14:31:22 4 of 4 PASS dbt_utils_source_equality_bigquery_external_people_csv_unpartitioned_id__first_name__last_name__email__ref_people_ [PASS in 1.64s]
14:31:22
14:31:22 Finished running 4 tests in 0 hours 0 minutes and 10.13 seconds (10.13s).
14:31:22
14:31:22 Completed successfully
14:31:22
14:31:22 Done. PASS=4 WARN=0 ERROR=0 SKIP=0 TOTAL=4
CircleCI received exit code 0
```

### System information
**The contents of your `packages.yml` file:**

**Which database are you using dbt with?**
- [ ] redshift
- [ ] snowflake
- [ ] other (specify: ____________)

**The output of `dbt --version`:**
```

```

**The operating system you're using:**

**The output of `python --version`:**

### Additional context

Couple of options I suppose:
1. We can probably make sure to drop table if exists as a run-operation prior to the `stage_external_sources` op.
2. Drop the external tables AFTER the test (?).
3. Add some "dynamic" strings to the external table name (e.g. the PR number).