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

tests/hghave.py

@@ -829,6 +829,17 b' def has_dev_full():'
     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:

tests/test-install.t

@@ -241,12 +241,15 b' the last test in this file, because we d'
 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,