Invalid test code when using a backend where workers is non-numeric, e.g. a character vector

[HenrikBengtsson]

$ Rscript -e future.tests::check --args --test-plan="cluster" --test-tags="plan,workers"
── Settings ──────────────────────────────────────────────────────────────────────────────────────────────────────────────
- future.tests version      : 0.6.0.9003
- R_FUTURE_TESTS_ROOT       : 
- Option 'future.tests.root': NULL
- Default test set folder   : /home/henrik/R/x86_64-pc-linux-gnu-library/4.3-CBI-gcc11/future.tests/test-db
- Max number of workers     : 2
- Timeout                   : N/A

── Running 3 test sets with plan(cluster) ────────────────────────────────────────────────────────────────────────────────
ℹ Backend package: future 1.32.0
✔  1. plan() - workers=<numeric> (1 test) (506ms)                               
✖  2. plan() - workers=<function> (1 test) (7ms)                                
  ✖ lazy=FALSE, globals=TRUE, stdout=TRUE
    Error of class ‘simpleError’ with message:
    invalid format '%g'; use format %s for character objects
    Call:
    sprintf("Number of workers according to plan(): %g\n", workers())
    Output:
    Number of initial workers: 2
✔  3. plan() - workers=<invalid> (1 test) (2ms)                                 

Number of tests: 3
Number of test steps: 3
Duration: 542ms
Results: 2 ok ✔ | 0 skips ★ | 1 error ✖ | 0 timeouts T