Fail if schema test coverage is below 100%, except on dev

[ralfhandl]

The schema-test-coverage.sh script will now exit with code 1 if schema coverage falls below 100%, except on the dev branch, where we have insufficient test cases.

This will prevent schema additions without accompanying test cases, which we recently had on v3.2-dev.

Tested with dummy PRs:

• Dummy 3.1 #4687 has green checks because test coverage is 100%
• Dummy 3.2 #4686 has red schema-test check because coverage is only 98%

• schema changes are included in this pull request
• schema changes are needed for this pull request but not done yet
• no schema changes are needed for this pull request