tests: add optional output when wheel is missing

If the `wheel` package isn't installed, pip will emit a warning
on stderr complaining about that. Alternatively, we could
redirect stderr to stdout and swallow the warning. But I like
tests having visibility of warnings, as it helps keep us honest
regarding edge cases.

Differential Revision: https://phab.mercurial-scm.org/D7371

tests/test-install.t

@@ -250,6 +250,7 b' On Python 2, we use the 3rd party virtua'
 Note: we use this weird path to run pip and hg to avoid platform differences,
 since it's bin on most platforms but Scripts on Windows.
   $ ./installenv/*/pip install --no-index $TESTDIR/.. >> pip.log
+    Failed building wheel for mercurial (?)
   $ ./installenv/*/hg debuginstall || cat pip.log
   checking encoding (ascii)...
   checking Python executable (*) (glob)