The libtest.la headers were installed in very weird place, in fact, we
don't need to list them in the HEADERS variable, listing them in SOURCES
is enough for autotools to figure out how to compile the convenience
library.
Move the libtest code into a 'libtest' subdirectory and make it
one of the SUBDIRS in the tests Makefile. having it at the top level
required having "." as one of the subdirs, and that caused the
unit tests to be executed twice.