gofix: make fix order explicit

Also test only specific fixes, not all fixes.
This means we don't have to keep updating old
test cases to match later changes to the library.

I had to adjust some of the reflect test cases,
because they were implicitly testing
reflect+oserrorstring, not just reflect.

R=golang-dev, bradfitz, r
CC=golang-dev
https://golang.org/cl/5283042
49 files changed