tests: Use env in bash scripts shebang

Bash can be in both /usr/bin/bash or /bin/bash (or elsewhere!) depending on
the distro, so let's be generic using env to figure it out.

This comes from a packaging lintian error we get:

 E: mutter-16-tests: wrong-path-for-interpreter /usr/bin/bash != /bin/bash
    [usr/share/mutter-16/tests/socket-launch.sh]

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/4288>

src/tests/clutter/interactive/meson/gen-test-unit-names.sh

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/usr/bin/env sh
 
 outputfile=$1
 shift

src/tests/kvm/run-kvm-test.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 set -e
 

src/tests/kvm/virtme-run.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 set -e
 

src/tests/socket-launch.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 export LISTEN_PID=$$
 exec "$@"

src/tests/x11-test.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 set -ex