Refactor license option tests to use Mock objects instead of lambdas

[kemzeb]

Using Mock objects makes the intent more clearer compared to how we were using lambdas.

[kemzeb]

Really need to dig into why the test coverage keeps failing in a non-deterministic manner.

Looking at the failure:

---------- coverage: platform linux, python 3.10.16-final-0 ----------
Name                                 Stmts   Miss Branch BrPart  Cover   Missing
--------------------------------------------------------------------------------
src/pipdeptree/_discovery.py            71      0     32      1    99%   80->70
src/pipdeptree/_freeze.py               51      7      6      3    82%   64-70, 77-79, 84-85
src/pipdeptree/_models/dag.py          129      1     50      2    98%   67->69, 143
src/pipdeptree/_models/package.py      138      7     34      3    93%   84, 139->141, 142, 190, 221-224
src/pipdeptree/_render/graphviz.py      52     19     18      3    63%   29-34, 41-43, 48-56, 64-66, 101-102
src/pipdeptree/_render/json.py           7      2      0      0    71%   22-23
src/pipdeptree/_render/mermaid.py       47      0     18      1    98%   62->60
src/pipdeptree/_validate.py             66      0     38      1    99%   85->88
tests/_models/test_package.py          132      1      0      0    99%   18
tests/conftest.py                       61     61     12      0     0%   1-95
tests/our_types.py                       2      2      0      0     0%   1-5
tests/test_cli.py                       94     94      2      0     0%   1-[162](https://github.com/tox-dev/pipdeptree/actions/runs/14076981932/job/39421605633?pr=467#step:8:163)
tests/test_detect_env.py                29     29      0      0     0%   1-54
tests/test_discovery.py                105    105      0      0     0%   1-186
tests/test_freeze.py                     8      8      0      0     0%   1-12
tests/test_non_host.py                  79     79      0      0     0%   1-159
tests/test_pipdeptree.py                19     19      0      0     0%   1-41
tests/test_validate.py                  30     30      0      0     0%   1-175
tests/test_warning.py                   10     10      0      0     0%   1-18
--------------------------------------------------------------------------------
TOTAL                                 [166](https://github.com/tox-dev/pipdeptree/actions/runs/14076981932/job/39421605633?pr=467#step:8:167)3    474    274     14    74%

18 files skipped due to complete coverage.
Coverage HTML written to dir /home/runner/work/pipdeptree/pipdeptree/.tox/3.10/tmp/htmlcov
Coverage XML written to file /home/runner/work/pipdeptree/pipdeptree/.tox/coverage.3.10.xml

FAIL Required test coverage of 88.0% not reached. Total coverage: 73.77%

I notice that the test coverage fails due to the top-level files in test/ being 0% covered (this should not be true). Throughout my time observing these failures this has always been the reason for these test coverage related failures.