testenv: introduce testsrcdir.cfg

Source directories can have more than one testenv.cfg file. Some options
have been added to testenv.cfg that are not really specific to a single
testsuite or how it gets executed, but to the whole source directory.

This is not ideal, because we need to have additional code that ensures
these options have the same value across all testenv.cfg files in the
same source directory, and we need functions that just pick the value
from the first of these configs. When we change such a value, we also
need to potentially make the change in multiple files.

Resolve this by introducing a new config file testsrcdir.cfg, that can
be optionally present in the source directory, and has options that
count for all testsuites in the same source directory.

Move max_jobs_per_gb_ram= as first option to the new file. The following
patches will move all other source directory specific options to
testsrcdir.cfg.

I have also considered naming the new file testdir.cfg, but the name
"testdir" is already used in the source code for the place where we
execute the individual testsuites after copying configs into that
directory.

Change-Id: I8eceea7b874ce1352e2cc9780b77d2a8e694cd28
---
M 5gc/testenv.cfg
A 5gc/testsrcdir.cfg
M _testenv/README.md
M _testenv/testenv.py
M _testenv/testenv/testenv_cfg.py
A _testenv/testenv/testsrcdir_cfg.py
M _testenv/testenv/testsuite.py
7 files changed, 105 insertions(+), 70 deletions(-)