[libbacktrace] Add tests for unused formats

[libbacktrace] Add tests for unused formats

When building libbacktrace, we typically use elf.c, and don't build pecoff.c,
xcoff.c or unknown.c.

Add testcases that use unused format to ensure that we also build and
test those on a typical development setup.

Bootstrapped and reg-tested on x86_64.

2018-11-30  Tom de Vries  <tdevries@suse.de>

	* Makefile.am (check_PROGRAMS): Add test_elf, test_xcoff_32,
	test_xcoff_64, test_pecoff and test_unknown.
	* Makefile.in: Regenerate.
	* test_format.c: New file.

From-SVN: r266668