✅ Fix decode utf-7 test for ruby 2.6

Because the `v0.3-bugfixes` branch doesn't have the latest version of
`core_assertions`, we can't use `assert_linear_performance`.  It's
enough for me that it *is* tested in the `master` branch, although that
won't test against ruby 2.6.

test/net/imap/test_imap_data_encoding.rb

@@ -30,9 +30,12 @@ def test_decode_utf7
     utf8 = "\357\274\241\357\274\242\357\274\243".dup.force_encoding("UTF-8")
     assert_equal(utf8, s)
 
-    assert_linear_performance([1, 10, 100], pre: ->(n) {'&'*(n*1_000)}) do |s|
-      Net::IMAP.decode_utf7(s)
-    end
+    # This assertion is not in the current version of core_assertions.  The fix
+    # has been backported to v0.3.5, but only tested in master and v0.4.0+.
+    #
+    # assert_linear_performance([1, 10, 100], pre: ->(n) {'&'*(n*1_000)}) do |s|
+    #   Net::IMAP.decode_utf7(s)
+    # end
   end
 
   def test_encode_date