test: generate the `test-sparse-revlog` artifact when slow-test is allowed

The `test-sparse-revlog.t` logic requires a large bundle to be generated. This
bundle can be reused from one run to the next but its initial generation is
slow. With this patch, if the bundle is missing and slow tests are permitted,
the bundle will be generated during the test run.

This should ensure that CI run this test.

tests/test-sparse-revlog.t

@@ -12,10 +12,22 b' repeatedly while some of it changes rare'
   $ bundlepath="$TESTDIR/artifacts/cache/big-file-churn.hg"
 
   $ expectedhash=`cat "$bundlepath".md5`
+
+#if slow
+
+  $ if [ ! -f "$bundlepath" ]; then
+  >     "$TESTDIR"/artifacts/scripts/generate-churning-bundle.py > /dev/null
+  > fi
+
+#else
+
   $ if [ ! -f "$bundlepath" ]; then
   >     echo 'skipped: missing artifact, run "'"$TESTDIR"'/artifacts/scripts/generate-churning-bundle.py"'
   >     exit 80
   > fi
+
+#endif
+
   $ currenthash=`f -M "$bundlepath" | cut -d = -f 2`
   $ if [ "$currenthash" != "$expectedhash" ]; then
   >     echo 'skipped: outdated artifact, md5 "'"$currenthash"'" expected "'"$expectedhash"'" run "'"$TESTDIR"'/artifacts/scripts/generate-churning-bundle.py"'