Allow ENV to override GlobalState#test_library

In our project we have a very small amount of rspec tests and a large
amount of minitest tests. The test detection method prefers rspec over
minitest and as a result, the testing code lenses don't work for the
vast majority of our tests.

This change allows a user to set an ENV variable to force the
test_library to be what they choose.

lib/ruby_lsp/global_state.rb

@@ -110,7 +110,9 @@ def detect_formatter
 
     sig { returns(String) }
     def detect_test_library
-      if direct_dependency?(/^rspec/)
+      if ENV.key?("RUBY_LSP_TEST_LIBRARY")
+        ENV["RUBY_LSP_TEST_LIBRARY"]
+      elsif direct_dependency?(/^rspec/)
         "rspec"
       # A Rails app may have a dependency on minitest, but we would instead want to use the Rails test runner provided
       # by ruby-lsp-rails. A Rails app doesn't need to depend on the rails gem itself, individual components like

test/global_state_test.rb

@@ -47,6 +47,20 @@ def test_detects_rspec_if_both_rails_and_rspec_are_present
       assert_equal("rspec", GlobalState.new.test_library)
     end
 
+    def test_detects_library_from_env_if_present
+      original_env = ENV.fetch("RUBY_LSP_TEST_LIBRARY", :not_present)
+      ENV["RUBY_LSP_TEST_LIBRARY"] = "minitest"
+      stub_dependencies("test-unit" => "1.2.3")
+      stub_dependencies("rspec" => "1.2.3")
+      assert_equal("minitest", GlobalState.new.test_library)
+    ensure
+      if original_env == :not_present
+        ENV.delete("RUBY_LSP_TEST_LIBRARY")
+      else
+        ENV["RUBY_LSP_TEST_LIBRARY"] = original_env
+      end
+    end
+
     def test_direct_dependency_returns_false_outside_of_bundle
       File.expects(:file?).at_least_once.returns(false)
       stub_dependencies({})