fix: correct single-precision CI testing

[sbryngelson]

Closes #759

Summary

• Add \$PRECISION to the Test step in `.github/workflows/test.yml` so single-precision binaries are actually tested with `--single` tolerances (root bug: `$PRECISION` was only in the Build step env, so the Test step ran double-precision test selection/tolerance against single-precision output)
• Change single-precision CI job from `mpi: no-mpi` to `mpi: mpi` so `--test-all` is passed and a broader test set is exercised

Test plan

• CI single-precision job builds with `--mpi --single`
• CI single-precision job runs tests with `--single` flag (1e-1 tolerances)
• No regressions in regular (double-precision) CI jobs

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📥 Commits

Reviewing files that changed from the base of the PR and between d513442 and 84ffb7a.

📒 Files selected for processing (1)

• .github/workflows/test.yml

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[github-actions]

Claude Code Review

Head SHA: 84ffb7a

Files changed:

• 1
• .github/workflows/test.yml

Findings

Implicit --test-all side effect from mpi: no-mpi → mpi: mpi

The matrix entry changed at line 175 (mpi: no-mpi → mpi: mpi) silently activates --test-all for the single-precision run via the existing conditional:

TEST_ALL: ${{ matrix.mpi == 'mpi' && '--test-all' || '' }}

Before this PR, the single-precision entry had mpi: no-mpi, so TEST_ALL evaluated to '' and the test command ran only the default subset. After this change, TEST_ALL becomes '--test-all', so the effective test command becomes:

/bin/bash mfc.sh test -v --max-attempts 3 -j $(nproc) $ONLY_CHANGES --test-all --single

This is a substantially larger test scope than before. If the intent was only to add --single precision flag coverage with MPI, verify that running the full --test-all suite in this job is also desired — if not, the TEST_ALL condition should be updated to decouple MPI presence from full-suite execution.