tests: use !/usr/bin/env bash consistently

This adjustment was last done in f96bc66901,
but it seems some files were missed and some were added later.

test/TEST-38-FREEZER/test.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 # SPDX-License-Identifier: LGPL-2.1-or-later
 set -e
 

test/TEST-52-HONORFIRSTSHUTDOWN/test.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 # SPDX-License-Identifier: LGPL-2.1-or-later
 set -e
 

test/units/testsuite-15.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 # SPDX-License-Identifier: LGPL-2.1-or-later
 set -eux
 set -o pipefail

test/units/testsuite-22.11.sh

@@ -1,4 +1,4 @@
-#! /bin/bash
+#!/usr/bin/env bash
 # SPDX-License-Identifier: LGPL-2.1-or-later
 set -e
 set -x

test/units/testsuite-22.12.sh

@@ -1,4 +1,4 @@
-#! /bin/bash
+#!/usr/bin/env bash
 # SPDX-License-Identifier: LGPL-2.1-or-later
 #
 # Test the "Age" parameter (with age-by) for systemd-tmpfiles.

test/units/testsuite-52.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 # SPDX-License-Identifier: LGPL-2.1-or-later
 set -ex
 set -o pipefail