fix mock util

[rfbgo]

develop is currently broken due to an unfortunate serious of merges. #1747 removed something #1802 relied up, but I was very slow merging in #1747 so we have an order of operations issues:

Order of events:

1. Add explicit version bootstrapping #1747's branch forked from develop at 0220c79 (Refactor to avoid hard-coding style exemptions #1798). Its new test_utility_base.py set provided_executables and similar fields as plain class attributes on _MockUtility.
   That worked at the time.
2. Directive followup #1802 was merged into develop (07414a3). It deliberately removed _class_directive_values, the support that let a subclass set those fields as plain class attributes. Nothing
   already in develop relied on it, so Directive followup #1802's CI passed.
3. Add explicit version bootstrapping #1747 was merged last (0252411) without being retested against a develop that already had Directive followup #1802. Its branch tip passes 16/16 on its own. I confirmed that merging Directive followup #1802, or
   anything after it, into that tip makes the same 3 tests fail. Merging only PRs from before Directive followup #1802 keeps it passing.

[ramble-project-pr-bot]

Ramble Performance Test Metrics

Results produced with commit: 3559a9f

Test Name	Outcome	Duration (s)	Most Recent Run (s)	Last 5 Avg (s)
test_analyze_large_file	passed	1.0748	1.0493 (0252411)	1.0421
test_large_template_expansion	passed	1.2143	1.2075 (0252411)	1.1997
test_many_experiments	passed	31.3953	30.7300 (0252411)	30.1016
test_many_objects_defaults	passed	17.4165	16.6550 (0252411)	16.2215
test_matrix_filter_perf	passed	1.1768	1.0847 (0252411)	1.1010

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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