tests: More work on error handling

tests/dirent/main.c

tests/expected/stdlib/rand.stdout

tests/expected/string/strrchr.stdout

tests/fcntl/create.c

tests/fcntl/fcntl.c

tests/stdio/freopen.c

tests/stdio/fseek.c

tests/stdio/popen.c

tests/stdio/rename.c

tests/stdlib/rand.c

tests/string/strrchr.c

tests/test_helpers.h

tests/unistd/link.c

tests/unistd/write.c