Makefile: introduce testpy-% target for testing with a specifc Python

This makes it easy to do 'make testpy-2.4.6 TESTFLAGS="-j 16"' and the
Makefile will build Python if needed and then run tests (with -j 16)
with the resulting Python.

You can set the environment variable HGPYTHONS to a nice location on
your machine to cache the Python builds globally. If that's not set,
it builds them inside build/pythons.

Makefile

@@ -7,6 +7,8 b''
 PREFIX=/usr/local
 export PREFIX
 PYTHON=python
+$(eval HGROOT := $(shell pwd))
+HGPYTHONS ?= $(HGROOT)/build/pythons
 PURE=
 PYFILES:=$(shell find mercurial hgext doc -name '*.py')
 DOCFILES=mercurial/help/*.txt
@@ -98,6 +100,13 b' tests:'
 test-%:
 	cd tests && $(PYTHON) run-tests.py $(TESTFLAGS) $@
 
+testpy-%:
+	@echo Looking for Python $* in $(HGPYTHONS)
+	[ -e $(HGPYTHONS)/$*/bin/python ] || ( \
+	cd $$(mktemp --directory --tmpdir) && \
+        $(MAKE) -f $(HGROOT)/contrib/Makefile.python PYTHONVER=$* PREFIX=$(HGPYTHONS)/$* python )
+	cd tests && $(HGPYTHONS)/$*/bin/python run-tests.py $(TESTFLAGS)
+
 check-code:
 	hg manifest | xargs python contrib/check-code.py