x/text: requires explicit (old) version of icu library accidentally

[japettyjohn]

What version of Go are you using (go version)?

1.8.3

What operating system and processor architecture are you using (go env)?

darwin amd64

What did you do?

Attempt to build a project with a dependency on collate/tools/colcmp/icu.go with the icu tag.

What did you expect to see?

Clean build.

What did you see instead?

Failure to find icu version 57 because of the hard dependency from https://github.com/golang/text/blob/master/cases/icu.go (which as noted is only for tests but is not a test file due to the cgo). This could be changed to match the same versionless dependency like in the collate file.

[odeke-em]

/cc @mpvl