Fix hardcoded /tmp paths in testing (bug 13888)

[Florian Weimer]

* Joseph Myers:

> +CFLAGS-bug3.c += -DOBJPFX=\"$(objpfx)\"
> +CFLAGS-bug4.c += -DOBJPFX=\"$(objpfx)\"
> +CFLAGS-bug5.c += -DOBJPFX=\"$(objpfx)\"
> +CFLAGS-test-fseek.c += -DOBJPFX=\"$(objpfx)\"
> +CFLAGS-test-popen.c += -DOBJPFX=\"$(objpfx)\"
> +CFLAGS-test_rdwr.c += -DOBJPFX=\"$(objpfx)\"

There is some overlap here with DJ's test-in-container patch, which
also needs to find a test directory (although there it is
$(common-objpfx)).

Should we make this available in a more general way, perhaps as a
subdir-specific header file, or by changing the make rule for
compiling tests?