Add script testing that sdist result works

It was quite fiddly to get something which works both locally and,
looking ahead, on the Travis infrastructure.  See

    travis-ci/travis-ci#8589

tests/test-sdist.sh

@@ -0,0 +1,33 @@
+#!/usr/bin/env bash
+
+set -x
+
+PYTHON_VER=$(python3 -c 'if True:
+    import sys
+    v = sys.version_info
+    print(".".join(map(str,[v.major,v.minor,v.micro])))')
+
+OPT_PYTHON=/opt/python/$PYTHON_VER/bin/python3
+if [ -x $OPT_PYTHON ]; then
+    PYTHON=$OPT_PYTHON
+else
+    PYTHON=$(which python3)
+fi
+
+D=$(mktemp -d)
+function rm_tmpdir {
+    rm -rf "$D"
+}
+trap rm_tmpdir EXIT
+
+SRCDIR=$(pwd)
+
+python3 setup.py sdist -d $D \
+    && $PYTHON -m venv $D \
+    && cd / \
+    && ( source $D/bin/activate \
+             && python3 -m ensurepip \
+             && pip3 install -r "$SRCDIR"/requirements.txt \
+             && pip3 install $D/gitdendrify-*.tar.gz ) \
+    && echo Success
+