Add shebang to test shell scripts

This ensures that the test scripts are correctly run with the Bourne shell, regardless of the (potentially more exotic) shell that is used to invoke the script.

test/asm/test.sh

@@ -1,3 +1,4 @@
+#!/bin/sh
 fname=$(mktemp)
 rc=0
 

test/asm/update-refs.sh

@@ -1,3 +1,4 @@
+#!/bin/sh
 fname=$(mktemp)
 
 for i in *.asm; do

test/link/test.sh

@@ -1,3 +1,4 @@
+#!/bin/sh
 otemp=$(mktemp)
 gbtemp=$(mktemp)
 gbtemp2=$(mktemp)

test/link/update-refs.sh

@@ -1,3 +1,4 @@
+#!/bin/sh
 otemp=$(mktemp)
 gbtemp=$(mktemp)