run-tests: fix _checkhglib() so it's correct when using --with-hg.
Since this makes the warning less likely, upgrade it to a "real"
warning to stderr.
#!/bin/sh
hg init a
cd a
hg log
hg grep wah
hg manifest
hg verify
ls .hg
ls .hg/store
cd ..
hg clone a b
cd b
hg verify
ls .hg
ls .hg/store