Forward NEXTEST_TEST_JOBS to the dev-test target

[leynos]

make test honours NEXTEST_TEST_JOBS, and make dev-test silently drops it.

test-nextest forwards both $(NEXTEST_BUILD_JOBS) and $(NEXTEST_TEST_JOBS)
to cargo nextest run. dev-test, the local accelerated runner, forwards only
$(NEXTEST_BUILD_JOBS). Both targets invoke cargo nextest run, so a
contributor who sets NEXTEST_TEST_JOBS sees it applied by one and ignored by
the other. Separating the build-worker bound from the test-worker bound exists
precisely to remove that surprise.

Raised by CodeRabbit while reviewing #664, which moved Linux CI to Ubicloud and
Windows and macOS to GitHub-hosted runners. It was deliberately left out of
that pull request: dev-test is a local workflow that no CI lane exercises, so
changing it inside a runner migration would put an untested change behind a
green signal that never runs it. CodeRabbit agreed with that scoping and asked
for this follow-up.

Fix: forward $(NEXTEST_TEST_JOBS) from dev-test as well, and cover the
agreement between the two targets in the Makefile contract tests so they cannot
drift again.