test-read-dwarf: ensure in_elf_path exists and add missing test files

test-read-dwarf: ensure in_elf_path exists and add missing test files

test-read-dwarf silently succeeded even if the input elf file was not
existing. Hence, make distcheck succeeded even though the testfiles were
not distributed. Assert on the existence of the input file and add the
missing test case files.

	* tests/data/Makefile.am: add missing test case files
	* tests/test-read-dwarf.cc (test_task::perform): assert the
	input elf file exists.

Signed-off-by: Matthias Maennich <maennich@google.com>