Some of the _test.go files in the encoding packages contain a private
function called testEqual that calls testing.Errorf if the arguments
passed to it are unequal. The line numbers output by such calls to
Errorf identify the failure as being in testEqual itself which is not
very useful. This commit fixes the problem by adding a call to the
new t.Helper method in each of the testEqual functions. The line
numbers output when errors do occur now identify the real source of
the error.