Update some more recent tests to use /usr/bin/env to find bash

The recentl-applied patch had grown stale, so update the tests that had
been created since it was originally written.

test/emacs-large-search-buffer

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 test_description="Emacs with large search results buffer"
 . test-lib.sh
 

test/multipart

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 test_description="output of multipart message"
 . ./test-lib.sh
 

test/search-by-folder

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 test_description='"notmuch search" by folder: (with variations)'
 . ./test-lib.sh
 

test/search-insufficient-from-quoting

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 test_description='messages with unquoted . in name'
 . ./test-lib.sh
 

test/search-position-overlap-bug

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 # Test to demonstrate a position overlap bug.
 #