TEST-80: synchronize explicitly instead of by time

This removes "sleep" invocations, and makes the notify access testcase a
lot more robust to runtime jitter. We use a pair of fifos in the fs to
sync instead.

Also various other improvoements, including comments.

(Also removes the unnecessary "no-qemu" restriction)

test/TEST-80-NOTIFYACCESS/test.sh

@@ -3,7 +3,6 @@
 set -e
 
 TEST_DESCRIPTION="test NotifyAccess through sd-notify"
-TEST_NO_QEMU=1
 
 # shellcheck source=test/test-functions
 . "${TEST_BASE_DIR:?}/test-functions"