* test: change system-test setup against emulator

Use Docker image instead of manually downloading the emulator binary and
running it.

* chore: cleanup unusued script file

* fix: always use the latest emulator version

* perf: increase default min sessions to 25

* fix: also ignore 'Instance not found' on fill

* tests: add test for other error than NOT_FOUND

* docs: fix function documentation

Co-authored-by: skuruppu <skuruppu@google.com>

* docs: fix function documentation

Co-authored-by: skuruppu <skuruppu@google.com>

Co-authored-by: skuruppu <skuruppu@google.com>

Getting a session would set a new event listener on the session pool that
would not always be removed. The event listener is now removed after each
call.

Co-authored-by: skuruppu <skuruppu@google.com>

🤖 I have created a release \*beep\* \*boop\* 
---
### [5.2.1](https://www.github.com/googleapis/nodejs-spanner/compare/v5.2.0...v5.2.1) (2020-07-07)


### Bug Fixes

* remove error listener to prevent memory leak ([#1168](https://www.github.com/googleapis/nodejs-spanner/issues/1168)) ([523bd67](https://www.github.com/googleapis/nodejs-spanner/commit/523bd67ed6d5ecbfe9abce0f1b6ed4cce2c07b30))


### Performance Improvements

* increase default min sessions to 25 ([#1167](https://www.github.com/googleapis/nodejs-spanner/issues/1167)) ([e4aba27](https://www.github.com/googleapis/nodejs-spanner/commit/e4aba27d307e5932b223121af5bea37a7418bb20))
---


This PR was generated with [Release Please](https://github.com/googleapis/release-please).