Add FIPS spec helper

Adds a helper for running specs under FIPS mode.

spec/support/fips.rb

+# frozen_string_literal: true
+# rubocop: disable RSpec/EnvAssignment
+
+RSpec.configure do |config|
+  config.around(:each, :fips_mode) do |example|
+    prior_value = ENV["FIPS_MODE"]
+    ENV["FIPS_MODE"] = "true"
+
+    example.run
+
+    ENV["FIPS_MODE"] = prior_value
+  end
+end
+
+# rubocop: enable RSpec/EnvAssignment