Rename package fwk/testdata (Go reserved name)

[dolmen]

testdata is a special name for the Go toolchain. go help packages says:

Directory and file names that begin with "." or "_" are ignored by the go tool, as are directories named "testdata".

Package go-hep.org/x/hep/fwk/testdata should be renamed to avoid problems.

[dolmen]

This issue blocks any further changes I would like to propose to the fwk code as files in the fwk/testdata package would be affected.

[sbinet]

yeah, the code under that testdata "fake" package was written before the "internal" concept existed.
I guess one could move that code under, say, "fwk/internal/fwktest" or something.
WDYT?