* t/python8.sh, t/python9.sh: Merge into ...
* t/python-am-path-iftrue.sh: ... this new test, with minor adjustments.
* t/python4.sh, t/python5.sh, t/python6.sh, t/python7.sh: Merge into ...
* t/python-missing.sh: ... this new test.
* t/python5b.sh: Rename ...
* t/python-too-old.sh: ... like this, and adjust/extend.
* t/list-of-tests.mk: Adjust.