test fixtures: use /bin/sh, fix case on system without bash where tests would fail because interpretter not found

Author: tony

tmuxp/testsuite/fixtures/script_complete.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/bin/sh
 
 echo hello
 echo $?    # Exit status 0 returned because command executed successfully.

tmuxp/testsuite/fixtures/script_failed.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/bin/sh
 
 
 echoerr() { echo "$@" 1>&2; }