Fix "test_before_across_chunks" test

[stanislavlevin]

Pipe like "openssl rand -base64 2097152 | head -n 500" leads to "Broken pipe"
error on some systems. This is an unexpected behavior. So errors must be
suppressed.

Fixes: #523

[stanislavlevin]

Hello, how can i fix coverage?
Please advise.

[bmwiedemann]

IMHO the coveralls result is bogus, because it does not recognize that the change is in a test (and tests always cover themselves in addition to the code under test)

I independently discovered and submitted the same patch with this description:

Without this patch, the test failed with openssl-1.0.2p
because 502 lines were captured with the extra lines being at the end:

140510355240592:error:02012020:system library:fflush:Broken pipe:bss_file.c:434:fflush()
140510355240592:error:20074002:BIO routines:FILE_CTRL:system lib:bss_file.c:436:

neither openssl-1.0.2j nor 1.1.0h needed this patch.

I also learned this:
The changes around the fflushing were intentional:
openssl/openssl#3348

It was a fix for:
openssl/openssl#3232

[takluyver]

Thanks, seems reasonable. I don't know why coveralls reckons the coverage decreased, but it's only by a tiny amount, so I'm just going to ignore it.