tests: look for ensurepip before using venv stable

tests: look for ensurepip before using venv

Debian appears to cripple the venv module by default by
removing the associated ensurepip functionality. (The module
isn't present at all.) This caused test-install.t to fail when
using the Debian python3 unless the python3-venv package was
installed.

This commit introduces a new hghave requirement for detecting
ensurepip and makes the Python 3 install variant conditional on
its presence. This should make test-install.t pass when
using an incomplete Debian Python.

Differential Revision: https://phab.mercurial-scm.org/D7228

--- a/tests/hghave.py	Thu Oct 17 16:46:13 2019 -0400
+++ b/tests/hghave.py	Mon Nov 04 23:42:18 2019 -0800
@@ -829,6 +829,17 @@
     return os.path.exists('/dev/full')
 
 
+@check("ensurepip", "ensurepip module")
+def has_ensurepip():
+    try:
+        import ensurepip
+
+        ensurepip.bootstrap
+        return True
+    except ImportError:
+        return False
+
+
 @check("virtualenv", "Python virtualenv support")
 def has_virtualenv():
     try:

--- a/tests/test-install.t	Thu Oct 17 16:46:13 2019 -0400
+++ b/tests/test-install.t	Mon Nov 04 23:42:18 2019 -0800
@@ -241,12 +241,15 @@
 shell environment in order to make the virtualenv work reliably.
 
 On Python 3, we use the venv module, which is part of the standard library.
+But some Linux distros strip out this module's functionality involving pip,
+so we have to look for the ensurepip module, which these distros strip out
+completely.
 On Python 2, we use the 3rd party virtualenv module, if available.
 
   $ cd $TESTTMP
   $ unset PYTHONPATH
 
-#if py3
+#if py3 ensurepip
   $ "$PYTHON" -m venv installenv >> pip.log
 
 Note: we use this weird path to run pip and hg to avoid platform differences,