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
