Also now permit interactivly running tests without explicitly setting
$srcdir. This now works if we are inside ./tests and fails, as before,
when we are in a different directory.
Detected by shellcheck via CodeFactor.io
This is required to support parallel test runs.
Among others, make thise files dynamic:
* test-spool
* rsyslog.input
* rsyslog.out*.log
* tmp.in
Also:
* convert presort test statement to function
* cleanup imfile truncation test
* cleanup imfile-growing-file-id test
some cruft was left due to copy and paste error
* serialize mysql tests
