tests/narrow-library.sh

test: preserve the existing module policy config for testrepohg resuse

The test runner is, righfully, setting HGMODULEPOLICY to match the flavor
requests to run the tests. However this mess with the `testrepohg` ability to
run properly, as the newly set policy might be incompatible with its
installation, or its ability to read the test repo.

So we preserve the initial value in a dedicated variable and use it in the
`testrepohg` helper.

cat >> $HGRCPATH <<EOF
[extensions]
narrow=
[experimental]
changegroup3 = True
EOF