tests: Add missing portable Bash shebang line to script mcdc.sh

Symptom was:
> ./mcdc.sh: 61: [[: not found
> ./mcdc.sh: 70: Syntax error: "(" unexpected (expecting "then")

Author: hartwork

tests/lcov/mcdc/mcdc.sh

@@ -1,3 +1,4 @@
+#! /usr/bin/env bash
 CLEAN_ONLY=0
 COVER=