Fix incompatibility with rake 13.4+ verbose mode

Rake 13.4.0 (PR ruby/rake#394) introduced verbose console support by
setting ENV["TESTOPTS"] = "-v". Rake 13.4.2 (PR ruby/rake#723)
fixed this to preserve existing TESTOPTS values and append -v only
when not present.

However, ci_reporter_test_unit was adding the test_unit_loader.rb
file path to TESTOPTS, which caused conflicts:

1. Before rake 13.4.2: TESTOPTS was overwritten, breaking ci_reporter
2. After rake 13.4.2: TESTOPTS preserved, but appending -v caused
   rake_test_loader to interpret the loader file as a test file,
   resulting in "version unknown" errors

The fix uses RUBYOPT with Ruby's -r flag instead of TESTOPTS. This
properly requires the loader before tests run, avoiding the conflict
with rake's verbose handling.

Fixes compatibility with:
- Rake 13.4.0+ with Rake::TestTask verbose mode enabled
- Projects using t.verbose = true in Rakefile

See: ruby/rake#723
See: ruby/rake#394

Author: ogajduse

lib/ci/reporter/rake/test_unit.rb

@@ -5,7 +5,11 @@
     task :testunit do
       rm_rf ENV["CI_REPORTS"] || "test/reports"
       test_loader = CI::Reporter.maybe_quote_filename "#{File.dirname(__FILE__)}/test_unit_loader.rb"
-      ENV["TESTOPTS"] = "#{ENV["TESTOPTS"]} #{test_loader}"
+      # Use RUBYOPT instead of TESTOPTS to avoid conflicts with rake 13.4+
+      # which now preserves TESTOPTS and appends -v when verbose is enabled.
+      # Adding a file path to TESTOPTS caused rake_test_loader to treat the
+      # loader file as a test file, resulting in "version unknown" errors.
+      ENV["RUBYOPT"] = "#{ENV["RUBYOPT"]} -r#{test_loader}"
     end
   end
 end