All packages place testdata in a specific directory with the name
"testdata". The mime and strconv packages have been updated to use
the same convention.

mime: Move "mime/test.types" to "mime/testdata/test.types". Update test
code accordingly.

strconv: Move "strconv/testfp.txt" to "strconv/testdata/testfp.txt".
Update test code accordingly.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/7098072