Do not call bin/tools/genrandom unconditionally in system tests

$RANDFILE, i.e. bin/tests/system/random.data, should only be written to
if a system test requires support for cryptography and that file does
not already exist.  Otherwise, when multiple system tests are run in
parallel, that file might get truncated due to bin/tools/genrandom.c
using fopen() with mode "w" when writing the destination file.  With
unfortunate timing, this may cause system tests employing BIND tools
which need entropy (e.g. dnssec-keygen) to fail.

Make sure bin/tests/system/metadata/tests.sh no longer calls
bin/tools/genrandom since $RANDFILE is guaranteed to already be created
by the time bin/tools/genrandom is currently called because
bin/tests/system/metadata/prereq.sh uses bin/tests/system/testcrypto.sh.

Make sure bin/tests/system/sfcache/prereq.sh only writes to $RANDFILE if
it does not already exist.
15 jobs for 477-fix-intermittent-mkeys-system-test-failures in 9 minutes and 59 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Precheck
passed precheck:debian:sid:amd64 #44911
amd64 docker linux

00:00:15

 
  Build
passed build:centos:centos6:amd64 #44912
amd64 docker linux

00:04:30

passed build:centos:centos7:amd64 #44913
amd64 docker linux

00:03:27

passed build:debian:sid:amd64 #44914
amd64 docker linux

00:02:18

passed build:debian:sid:i386 #44915
i386 docker linux

00:04:05

 
  Test
passed install:debian:sid:amd64 #44924
amd64 docker linux

00:02:58

passed systemtest:centos:centos6:amd64 #44920
amd64 docker linux

00:04:56

passed systemtest:centos:centos7:amd64 #44921
amd64 docker linux

00:04:56

passed systemtest:debian:sid:amd64 #44922
amd64 docker linux

00:04:52

passed systemtest:debian:sid:i386 #44923
i386 docker linux

00:05:12

passed unittest:centos:centos6:amd64 #44916
amd64 docker linux

00:01:09

passed unittest:centos:centos7:amd64 #44917
amd64 docker linux

00:00:59

passed unittest:debian:sid:amd64 #44939
linux docker amd64

00:00:46

passed unittest:debian:sid:i386 #44919
i386 docker linux

00:00:56

failed unittest:debian:sid:amd64 #44918
amd64 docker linux

00:01:07