schema test coverage: return code 1 if coverage is below 100%

[ralfhandl]

This will cause the schema-test workflow to fail if the schema test coverage drops.

Fixes #107

[lornajane]

Adding some notes as I go along:

• the action that runs this is schema-tests
• that in turn runs npm test
• tested it by removing enough tests to reduce the coverage number and confirming that npm test on the main branch returns exit code 0 but on this branch I get exit code 1 as described.

[lornajane]

Thank you, a small but mighty change!