json.cc: use SELFTEST_LOCATION in selftests
gcc/ChangeLog:
* json.cc (selftest::assert_print_eq): Add "loc" param and use
ASSERT_STREQ_AT.
(ASSERT_PRINT_EQ): New macro.
(selftest::test_writing_objects): Use ASSERT_PRINT_EQ to capture
source location of assertion.
(selftest::test_writing_arrays): Likewise.
(selftest::test_writing_float_numbers): Likewise.
(selftest::test_writing_integer_numbers): Likewise.
(selftest::test_writing_strings): Likewise.
(selftest::test_writing_literals): Likewise.
Signed-off-by: David Malcolm <dmalcolm@redhat.com>