kvdb-web tests are failing spuriously on linux

[ordian]

Example job: https://travis-ci.org/paritytech/parity-common/jobs/632311169

Running headless tests in Chrome on `http://127.0.0.1:40119/`

Try find `webdriver.json` for configure browser's capabilities:

Not found

failed to detect test as having been run

output div contained:

    running 7 tests

console.log div contained:

    Upgrading or creating the database to version None, columns 1

    Previous version: 1, columns 0

driver status: signal: 9

driver stdout:

    Starting ChromeDriver 2.46.628388 (4a34a70827ac54148e092aafb70504c4ea7ae926) on port 40119

    Only local connections are allowed.

    Please protect ports used by ChromeDriver and related test frameworks to prevent access by malicious code.

driver stderr:

    [1578064198.493][SEVERE]: bind() failed: Cannot assign requested address (99)

[NikVolf]

The reason might be that they are run in parallel

[ordian]

could be related to rustwasm/wasm-bindgen#882