[test] Fix using suppressed test cases with interrupt-budget fuzzer

The suppression flag must be set before loading the tests to have an effect.
This got turned around in https://crrev.com/c/899366 and is fixed again by
this CL.

TBR=sergiyb@chromium.org
NOTRY=true

Bug: v8:6917
Change-Id: Ie58465288a0d6eec9a99a23d610710de9e1cdddd
Reviewed-on: https://chromium-review.googlesource.com/911114Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51209}

tools/testrunner/num_fuzzer.py

@@ -169,16 +169,11 @@ class NumFuzzer(base_runner.BaseTestRunner):
     suites = super(NumFuzzer, self)._load_suites(names, options)
     if options.combine_tests:
       suites = [s for s in suites if s.test_combiner_available()]
-    return suites
-
-  def _prepare_suites(self, suites, options):
-    """Sets additional configurations on test suites based on options."""
-    super(NumFuzzer, self)._prepare_suites(suites, options)
-
     if options.stress_interrupt_budget:
       # Changing interrupt budget forces us to suppress certain test assertions.
       for suite in suites:
         suite.do_suppress_internals()
+    return suites
 
   def _create_combiner(self, rng, options):
     if not options.combine_tests: