tests: add test for empty CA certs file

smf reported that an environment with no loaded CA certs resulted
in a weird error. I'd like to detect this a bit better so we can
display an actionable error message.

The actual error being globbed over in this patch is "unknown error"
with a ssl.c line number. That isn't useful at all.

tests/test-https.t

@@ -180,6 +180,14 b' variables in the filename'
   searching for changes
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+empty cacert file
+
+  $ touch emptycafile
+  $ hg --config web.cacerts=emptycafile -R copy-pull pull
+  pulling from https://localhost:$HGPORT/
+  abort: error: * (glob)
+  [255]
+
 cacert mismatch
 
   $ hg -R copy-pull pull --config web.cacerts="$CERTSDIR/pub.pem" \