tests: fix simplecov handling in integration tests

Closes Homebrew/homebrew#48250.

Signed-off-by: Baptiste Fontaine <batifon@yahoo.fr>

Library/Homebrew/test/test_integration_cmds.rb

@@ -30,7 +30,14 @@ class IntegrationCommandTests < Homebrew::TestCase
       -rconfig
       -rintegration_mocks
     ]
-    cmd_args << "-rsimplecov" if ENV["HOMEBREW_TESTS_COVERAGE"]
+    if ENV["HOMEBREW_TESTS_COVERAGE"]
+      # This is needed only because we currently use a patched version of
+      # simplecov, and gems installed through git are not available without
+      # requiring bundler/setup first. See also the comment in test/Gemfile.
+      # Remove this line when we'll switch back to a stable simplecov release.
+      cmd_args << "-rbundler/setup"
+      cmd_args << "-rsimplecov"
+    end
     cmd_args << (HOMEBREW_LIBRARY_PATH/"../brew.rb").resolved_path.to_s
     cmd_args += args
     Bundler.with_original_env do