✅ Mark assert_linear_performance test as pending

This is marked as pending in the backports branch because test-unit-ruby-core from git (added by #151) doesn't work with ruby 2.6.

Author: nevans

test/net/imap/test_imap_authenticators.rb

@@ -169,6 +169,7 @@ def test_digest_md5_authenticator_no_qop
  end
 
  def test_digest_md5_authenticator_illinear
+ pend "Need to backport #151 for assert_linear_performance"
  pre = ->(n) {'qop="a' + ',x'*n}
  assert_linear_performance([5, 10, 15, 20], pre: pre) do |challenge|
  auth = digest_md5("user", "pass")