Revert "tests: Fix path to `extcmd_test'"

This reverts commit 4054bcf1011c36f3316292bb4fd892a987ab9ca7.

The changed path to `extcmd_test' is actually invalid because `extcmd_test' is located beneath `<icingaweb>/modules/test/bin'.
I erroneously assumed the latter as invalid because the compiled `extcmd_test' was missing.