Add assertion messages

This message could be generated by `assert_predicate`, https://github.com/rails/rails/commit/1853a4f2c8b2bddfbde6aae80abb08310295201c#commitcomment-20546113
but I'd rather handwrite the message string if I could reduce magic from the code by doing so.

activerecord/test/cases/adapters/postgresql/infinity_test.rb

@@ -30,7 +30,7 @@ class PostgresqlInfinity < ActiveRecord::Base
     record = PostgresqlInfinity.new(float: "-Infinity")
     assert_equal(-Float::INFINITY, record.float)
     record = PostgresqlInfinity.new(float: "NaN")
-    assert record.float.nan?
+    assert record.float.nan?, "Expected #{record.float} to be NaN"
   end
 
   test "update_all with infinity on a float column" do

activerecord/test/cases/adapters/postgresql/numbers_test.rb

@@ -31,7 +31,7 @@ def test_values
     assert_equal 123456.789, first.double
     assert_equal(-::Float::INFINITY, second.single)
     assert_equal ::Float::INFINITY, second.double
-    assert third.double.nan?
+    assert third.double.nan?, "Expected #{third.double} to be NaN"
   end
 
   def test_update