The script allocfail.sh was being incorrectly removed by "make clean".
This patch fixes the problem.  This fixes
https://github.com/ianlancetaylor/libbacktrace/issues/81.  Ran
libbacktrace "make check" and "make clean" on x86_64-pc-linux-gnu.
Committed to mainline.

Ian

For https://github.com/ianlancetaylor/libbacktrace/issues/81
* Makefile.am (MAKETESTS): New variable split out of TESTS.
(CLEANFILES): Replace TESTS with BUILDTESTS and MAKETESTS.
* Makefile.in: Regenerate.