tests: move to t, allow yath test from root

By moving the tests subdirectory to t, we gain the ability to run `yath
test` with no arguments from inside `nix develop` in the root of the
the repo.

(`nix develop` is necessary in order to set the proper env vars for
`yath` to find our test libraries.)

t/jobs/hg-update.sh

@@ -0,0 +1,24 @@
+#! /bin/sh
+set -e
+
+repo="$1"
+STATE_FILE=$(pwd)/.hg-state
+if test -e $STATE_FILE; then
+    state=$(cat $STATE_FILE)
+    test $state -gt 1 && state=0
+else
+    state=0;
+fi
+
+case $state in
+    (0) echo "::Create repo. -- continue -- updated::"
+    hg init hg-repo
+    touch hg-repo/hg-file
+    cd hg-repo
+    hg add hg-file
+    hg commit -m "add hg file" hg-file -u foobar
+    ;;
+    (*) echo "::End. -- stop -- nothing::" ;;
+esac
+
+echo $(($state + 1)) > $STATE_FILE