This new flag downloads a known version of Go and builds with it. This
is meant for environments where we can't easily upgrade the installed Go
version.

* .travis.yml: remove install step for PR test builders

We added this step originally to avoid re-building everything
for every test. go test has become much smarter in recent go
releases, so we no longer need to install anything here.

* travis, appveyor, build, Dockerfile: bump Go to 1.14.2

* travis, appveyor: force GO111MODULE=on for every build

* appveyor: bump to Go 1.12.8

* appveyor: bump to Go 1.12.9

* appveyor: bump to Go 1.12.6

* vendor/vendor.json: govendor fetch github.com/karalabe/usb/^

* travis, appveyor: bump to Go 1.11.4

* internal/build: revert comment changes

Tests are now included as a submodule. This should make updating easier
and removes ~60MB of JSON data from the working copy.

State tests are replaced by General State Tests, which run the same test
with multiple fork configurations.

With the new test runner, consensus tests are run as subtests by walking
json files. Many hex issues have been fixed upstream since the last
update and most custom parsing code is replaced by existing JSON hex
types. Tests can now be marked as 'expected failures', ensuring that
fixes for those tests will trigger an update to test configuration. The
new test runner also supports parallel execution and the -short flag.

This ensures 'make test' finds all errors that remote CI would find.
Go 1.7 vet reports a false positive in package log, add a workaround.

This simplifies the build and should speed it up a bit because the
standard library doesn't need to be cross compiled on the 32bit builder.