tests: don't mutate global structs in core scheduler tests

[tgross]

Some of the core scheduler tests need the maximum batch size for writes to be smaller than the usual structs.MaxUUIDsPerWriteRequest. But they do so by unsafely modifying the global struct, which creates test flakes in other tests.

Modify the functions under test to take a batch size parameter. Production code will pass the global while the tests can inject smaller values. Turn the structs.MaxUUIDsPerWriteRequest into a constant, and add a semgrep rule for avoiding this kind of thing in the future.

Discovered while trying to debug #16112, but this should get backported to all supported versions whereas that will be only to 1.4.x

[tgross]

New semgrep rule from this PR, without the patch to the tests:

$ semgrep scan -c ./.semgrep/protect_globals.yml
METRICS: Using configs from the Registry (like --config=p/ci) reports pseudonymous rule metrics to semgrep.dev.
To disable Registry rule metrics, use "--metrics=off".
Using configs only from local files (like --config=xyz.yml) does not enable metrics.

More information: https://semgrep.dev/docs/metrics

Scanning 1683 files.

Findings:

  nomad/core_sched_test.go
     semgrep.no-overriding-struct-globals
        Mutating global structs is never safe

       1853┆ structs.MaxUUIDsPerWriteRequest = 2
          ⋮┆----------------------------------------
       1896┆ structs.MaxUUIDsPerWriteRequest = 2
          ⋮┆----------------------------------------
       1930┆ structs.MaxUUIDsPerWriteRequest = 2
          ⋮┆----------------------------------------
       1932┆ structs.MaxUUIDsPerWriteRequest = originalMaxUUIDsPerWriteRequest

Some files were skipped or only partially analyzed.
  Scan was limited to files tracked by git.
  Partially scanned: 3 files only partially analyzed due to a parsing or internal Semgrep error
  Scan skipped: 1 files larger than 1.0 MB, 1 files matching .semgrepignore patterns
  For a full list of skipped files, run semgrep with the --verbose flag.

Ran 1 rule on 1683 files: 4 findings.