Replace IsEqualJson gtest predicate with EXPECT_JSON_EQ

EXPECT_JSON_EQ has better debugging capabilities that the old
EXPECT_PRED2(IsEqualJson,... since it shows the correct line
numbers and both expected and actual JSON objects.

Change-Id: Iaed2a07230f2bade5c8a036fb5a8daff05613896
Reviewed-on: https://weave-review.googlesource.com/1792
Reviewed-by: Vitaly Buka <vitalybuka@google.com>