Make sure VERBOSE variable is exported to testall sub-invocation.
Redmine #4822: Fix incorrect background invocation on Windows.

You cannot use shell backgrounding on Windows. And the daemon cannot
daemonize either, so the correct means is to let the promise run in
the background.
Redmine #4822: Fix incorrect sleep invocation on Windows.
Redmine #4822: Fix incorrect "tee" invocation on Windows.
Fix annoying error message when failing a test.

The maximum variable size is 4096, so set it to that instead. It
potentially won't display all the content, but it's only for display
anyway.