test: a number of fixes.

Details:
- reorder.go: delete p8.
  (Once expectation is changed per b/4627 it is identical to p1.)
- switch.go: added some more (degenerate) switches.
- range.go: improved error messages in a few cases.
- method.go: added tests of calls to promoted methods.

R=iant
CC=golang-dev
https://codereview.appspot.com/7306087