Move test plugins from /scripts to /scripts/test.
POTFILES.skip that directory.
Add a readme to scripts dir.
Revise readme in scripts/test
Rename test9.scm which tests the new byte type support in Scheme.
Install it in unstable build, it is an important test,
long and sophisticated.
Install contactsheet.scm as a test plugin in unstable build.
See test/meson.build for comments about its issue.
