[test][prefix] Add dummy documentation to valid assets

This will allow to do more tests in for example

beat/beat.editor>
2 jobs for prefix_documentation in 32 minutes and 4 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
failed #166184
docker-build
build_linux_36

00:32:03

65.0%
failed #166185
macosx
build_macosx_36

00:20:10

65.0%
 
Name Stage Failure
failed
build_macosx_36 Build
TOTAL                                3706   1307    65%
----------------------------------------------------------------------
Ran 301 tests in 829.458s

FAILED (SKIP=3, failures=1)
Tests failed for beat.core-1.8.1b0-py36h3fd4c5d_79.tar.bz2 - moving package to /Users/gitlab/builds/f6f95b82/0/beat/beat.core/miniconda/conda-bld/broken
WARNING:conda_build.build:Tests failed for beat.core-1.8.1b0-py36h3fd4c5d_79.tar.bz2 - moving package to /Users/gitlab/builds/f6f95b82/0/beat/beat.core/miniconda/conda-bld/broken
TESTS FAILED: beat.core-1.8.1b0-py36h3fd4c5d_79.tar.bz2
ERROR: Job failed: exit status 1
failed
build_linux_36 Build
TOTAL                                3706   1306    65%
----------------------------------------------------------------------
Ran 301 tests in 806.146s

FAILED (SKIP=3, failures=8)
Tests failed for beat.core-1.8.1b0-py36h3fd4c5d_74.tar.bz2 - moving package to /local/builds/0d638152/0/beat/beat.core/miniconda/conda-bld/broken
WARNING:conda_build.build:Tests failed for beat.core-1.8.1b0-py36h3fd4c5d_74.tar.bz2 - moving package to /local/builds/0d638152/0/beat/beat.core/miniconda/conda-bld/broken
TESTS FAILED: beat.core-1.8.1b0-py36h3fd4c5d_74.tar.bz2
ERROR: Job failed: exit status 1