test: adjust treegen for TMPDIR ending with slash

Our macOS runners have such a TMPDIR value. It breaks
`config-luatest/basic_test.lua`, because net.box seems unable to connect
a Unix domain socket using an URI with a double slash in the middle.

See the comment in the code for details.

NO_DOC=testing helper change
NO_CHANGELOG=see NO_DOC
NO_TEST=see NO_DOC