Wait for finished in abstract content cache when fetching with blocking

[rldhont]

Description

After trying to fix travis segfault with Abstract content cache: delete task later #32218
The commit uses waitForFinished with arbitrary setting maximum wait to 5 seconds.

Checklist

• Commit messages are descriptive and explain the rationale for changes
• Commits which fix bugs include Fixes #11111 at the bottom of the commit message
• I have read the QGIS Coding Standards and this PR complies with them
• New unit tests have been added for core changes
• I have run the scripts/prepare-commit.sh script before each commit
• I have evaluated whether it is appropriate for this PR to be backported, backport requests are left as label or comment

[nyalldawson]

ARguably a much nicer approach anyway :D

[rldhont]

@nyalldawson travis is happy: no segfault and all tests passed