I spoke too soon. In a debug build, this hangs forever during the
second iteration:

./python.exe Lib/test/regrtest.py -uall -R1:1 test_ssl

Adding -v, it looks like two iterations are carried out perfectly (one
must be a trial run, one the warm-up run), but the third run goes
beserk; the output ends like this:

testAsyncoreServer (test.test_ssl.ThreadedTests) ...
 server:  read b'over\n' from client
 server:  closed connection <ssl.SSLSocket object, fd=6, family=2,
type=1, proto=0>
 server:  read b'' from client
 server:  closed connection <ssl.SSLSocket object, fd=10, family=2,
type=1, proto=0>
 server:  read b'' from client
 server:  closed connection <ssl.SSLSocket object, fd=10, family=2,
type=1, proto=0>
.
. (the last two lines repeated forever)
.