This commit makes a number of changes that should make
TestLoad_FullConfig easier to work with, and make the test more like
real world scenarios.
* use separate files in testdata/ dir to store the config source.
Separate files are much easier to edit because editors can syntax
highlight json/hcl, and it makes strings easier to find. Previously
trying to find strings would match strings used in other tests.
* use the exported config.Load interface instead of internal NewBuilder
and BuildAndValidate.
* remove the tail config overrides, which are only necessary with
nonZero works.
freddygv