tests/libtest: Add lib1521 to nodist_SOURCES

Since 467da3af, lib1521.c is generated instead of checked in. According
to the commit message, the intention was to remove it from the tarball
as well. However, it is still present when running make dist. To remove
it, add it to nodist_lib1521_SOURCES. This also means there is no need
for the manually added dist-rule in the Makefile.

Also update CMakelists.txt to handle the fact that we now may have
nodist_SOURCES.