Fix tests/run.sh

Commit a944636c02 "bashified" tests/run.sh for some reason.
Afterwards, commit 4abd820051 fixed some of the fall-out.
However, there is still a problem left.

We have "set -e" in this script. Thus, whenever some command exits with
status 0, the script abort. When the variable errors is zero/unset, the
command "((errors++))" has exit status zero. Thus, this instruction
caused the shell script to abort. This was not intended.

Fix this by using "((++errors))" instead.

Signed-off-by: Uli Schlachter <psychon@znc.in>

tests/run.sh

@@ -228,7 +228,7 @@ for f in $tests; do
             grep -q -E '[Ee]rror|assertion failed' "$awesome_log"; then
         echo "===> ERROR running $f <==="
         grep --color -o --binary-files=text -E '.*[Ee]rror.*|.*assertion failed.*' "$awesome_log" || true
-        ((errors++))
+        ((++errors))
     fi
 done