Hi,

Whilst working on the Reproducible Builds effort [0], we noticed that
golang-gonum-v1-plot could not be built reproducibly.

This was because it ships a different binary package depending
on whether the tests are run or not; the testsuite was generating
10s of files that were being installed:

  /usr/share/gocode/src/gonum.org/v1/plot/palette/testdata/reverse.png

Your override_dh_install is already removing a bunch of files
that embed timestamps for reproducibility reasons, but perhaps if
none of this test data is needed in the binary package (?) then
we can simply not ship the testdata/ directory completely and not
play whack-a-mole with new files. Please see the attached patch for
how this might look.

 [0] https://reproducible-builds.org/


Regards,