User default marker for Python tests

This PR sets "not (long_running or broken)" as the default marker for Python test top-level targets. Thus:
```
./build.py -f components/tools/OmeroPy/build.xml integration
```
is the equivalent of:
```
./build.py -f components/tools/OmeroPy/build.xml integration -DMARK="not (long_running or broken)"
```
Setting any other marker will override this default. To run all tests the marker must be explicitly *unset*:
```
./build.py -f components/tools/OmeroPy/build.xml integration -DMARK=
```
or
```
./build.py -f components/tools/OmeroPy/build.xml integration -DMARK=""
```
Note that this means the default for targets that do not have broken or long running tests will also have this markers set by default but that should *do no harm*.

If this PR is merged it will need a further docs PR to clarify the usage.

--no-rebase