DBAL-1319 #917 Remove GIT submodule entries and use autoload-dev for the test suite

[doctrinebot]

Jira issue originally created by user @doctrinebot:

This issue is created automatically through a Github pull request on behalf of Tobion:

Url: #917

Message:

1. remove obsolete entries from gitmodules
2. composer.lock is not part of the repo, so its useless in gitattributes
3. DoctrineTestCase is not used and is a remnant pre-split
4. use autoload-dev completely instead of an init script, complement to Moved Doctrine\Tests namespace to composer autoload-dev #770 and add travis cache and fix apcu installation cache#98

[doctrinebot]

Comment created by @doctrinebot:

A related Github Pull-Request [GH-917] was merged:
#917

[github-actions]

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