Restore tests + enforce javac-compatible layout

[caverac]

Two earlier PRs added unit-test coverage:

• #96 — ci/quality-checks (CI scaffolding: Checkstyle/SpotBugs/Spotless + workflow)
• #97 — test/apg-coverage (catdata/apg/* tests)
• #100 — test/cql-fdm-coverage (catdata/cql/fdm/* tests)

All three were merged to master.

What went wrong

The tests broke Eclipse / bare-javac build!!
CQL uses a flat layout where src/ itself is the single source root (sources live at src/catdata/...). The new tests were placed under src/test/... (aiming to reproduce the folder structure of the project):

• Gradle, configured with two source roots, accepted this
• Eclipse (single src/ root) and bare javac from the project root flagged it as a path/package mismatch.

Solution

Tests moved next to the code they exercise, distinguished by file-name suffix (*Test.java, *TestHelpers.java) — e.g. src/catdata/cql/fdm/ImportAlgebraTest.java. Package declarations already matched, so no source edits were needed.

This PR also adds a build step to the ci, that way we can catch potential compiling issues before code is merged