Do not terminate default test runs on test failure

[Roland McGrath]

> Whether to exit with error status if there were FAILs, or only if there 
> were ERRORs, was one of my questions on the first version of this patch, 
> where the only previous comment was from Brooks preferring FAILs not to 
> cause an error status 
> <https://sourceware.org/ml/libc-alpha/2014-02/msg00301.html>.  I'm happy 
> either way; error exit for any FAILs is just two extra grep lines like 
> those ensuring error exit for any ERRORs.

The status quo is that 'make check' only exits zero if all tests passed.
So please preserve that.