add predictable path names for pytest runner results

The particular missing feature that I've used the most is that the old runner puts test results in a predictable place. Let's say I want to run the resolver test and examine the contents of resolver/ns3/named.run - with the legacy runner I can just do that, I don't have to figure out that this time the results are in resolver_tmp_pnxahq01. If there's a way to tell pytest to put the test results in the test directory - or even just in the same place for two consecutive runs - I haven't discovered it. I often script these things; having the directory name change on every run makes that unnecessarily difficult, so I switch back to legacy.run.sh and then it's fine.

#4246 (comment 394780)

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Add a symlink with a predictable path to the latest test run(s). It's a convenience thing that should allow easier inspection of the results.

A single system test may consist of multiple modules which are run separately, e.g. doth has:

<Module doth/tests_gnutls.py>
<Module doth/tests_sh_doth.py>
<Module doth/tests_sslyze.py>

which may be executed independently and end up in different temporary directories. The symlink should include a module name/identifier to differentiate between them.

I think creating a directory like bin/tests/system/_last_test_run where the stable symlinks could be placed should work well.

Related #3810, #4251 (closed)