tests: rework tests on AM_PATH_PYTHON * 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. Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>