Replace '/bin/sh' with '/bin/bash'

Some of the scripts use extensions that are specific to bash so replace
'/bin/sh' with '/bin/bash' to avoid bashism warnings.

tests/Makefile

@@ -102,7 +102,7 @@ run_tests: $(PROGS)
 	perl $(TESTS)
 
 run_tests_mingw: $(PROGS)
-	/bin/sh do_tests.mingw
+	/bin/bash do_tests.mingw
 
 astyle:
 	astyle -n --style=kr --indent-switches --add-brackets *.c

tests/all_funcs

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 
 function proceed {
   read -p "proceed ? [n] " response

tests/do_tests.mingw

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 
 echo "MinGW test script starting"
 

tests/mexpand

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 
 # utility to macro-expand a test program
 CC=gcc