I was experiencing multiple failures with make check on a fresh installation of CentOS 8.3.
The solution for me was to use mock instead of rpmbuild. (I was not building as root with rpmbuild.) With mock all tests were successful. 

Sent from my T-Mobile 4G LTE device

------ Original message------
From: J Lumby
Date: Sat, Jan 16, 2021 11:31 AM
To: dovecot@dovecot.org;
Cc:
Subject:dovecot 2.3.13 : make check FAILURE : Assert failed: buffer_append_full_file

After successfully building dovecot 2.3.13 (linux kernel 5.10.1 , 
glibc-2.32 , gcc-9.3)

make check had one failure as follows :

test-buffer-istream.c:54: Assert failed: buffer_append_full_file(result, 
TEST_FILENAME, SIZE_MAX, &error) == BUFFER_APPEND_READ_ERROR
test-buffer-istream.c:56: Assert failed: error != NULL && *error != '\0'
buffer_append_full_file .............................................. : 
FAILED

After a brief look at the referenced code in test-buffer-istream.c  ,  I 
can't find out what this FAIL really means or whether it is serious.

The assert is finding that the result of the call to 
buffer_append_full_file is not BUFFER_APPEND_READ_ERROR,   but it does 
not report what the result code actually was.    Maybe errno was 
actually 0 and call succeeded?   I don't know.   Nor can I see how to 
add some code to find out.

Can
 anyone offer any advice on what this means and/or how to investigate?

Cheers,  John Lumby