run test for platform under dev mode

[ZheSun88]

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Currently, platform tests are running in production mode only, we should test everything under two modes.

as we have discovered the issue with running V14+ application on safari 10 under dev mode https://github.com/vaadin/flow/issues/6817 . after the issue got fixed, we can

1. update the travis configuration to test under dev mode; (on going)
2. update platform snapshot version to test under production mode(done);
3. update platform JDK test running under production mode (done).
4. Enable safari 10 test after flow issue got solved #956 (on going)

[ZheSun88]

master branch has been done; 14.1 and 14.0 are coming.

[ZheSun88]

PRs have been made for master: https://github.com/vaadin/platform/pull/945 V14: https://github.com/vaadin/platform/pull/955 V14.1: https://github.com/vaadin/platform/pull/968

[ZheSun88]

All open PRs have been closed and merged.

platform test are still running under production mode..

[manolo]

At the same time consider why we still need this : https://github.com/vaadin/platform/blob/7add7a36e3f6b5126be9bff7e39d7793bbe1f214/vaadin-platform-test/src/main/java/com/vaadin/platform/test/CustomServletDeployer.java#L50

[manolo]

Consider an extra step for running vaadin-platform-tests in dev-mode, probaly only for snapshots in order not to penalize validation times for PRs

[manolo]

done in #3788