To avoid potentially executing a different Perl than used by the rest
of the build and tests.
Also to be more portable by not relying on shebang support, though these
particular tests require POSIX shell anyway.
Closes#19767
Only actually needed servers should be listed and none is then implied
if no servers are listed.
Outputs a warning if "none" is still set as a server.
Closes#18466
Also introduce 'notexists' for verifying that directory entries do not
exist after a test. Now an explicit supported feature instead of using
"funny" perl in postcheck.
Closes#15046
The threee tags `<name>`, `</name>` and `<command>` were frequently used
with a leading space that this removes. The reason this habbit is so
widespread in testcases is probably that they have been copy and pasted.
Hence, fixing them all now might curb this practice from now on.
Closes#12028
