build: make tests optional

Add a meson option -Dtests and --without-tests automake option
to disable the compilation of all available testcases.
This is useful for compiling projects with Flatpak such as
GNOME Control Center which disables all possible integrations since they
only need the DBus part of ModemManager.

Contributes to https://gitlab.gnome.org/GNOME/gnome-control-center/-/merge_requests/1392

data/Makefile.am

@@ -1,5 +1,12 @@
 
-SUBDIRS = . dispatcher-connection dispatcher-fcc-unlock tests
+SUBDIRS = \
+	. \
+	dispatcher-connection \
+	dispatcher-fcc-unlock
+
+if WITH_TESTS
+SUBDIRS += tests
+endif
 
 edit = @sed \
        -e 's|@sbindir[@]|$(sbindir)|g' \

data/meson.build

@@ -1,7 +1,9 @@
 # SPDX-License-Identifier: GPL-2.0-or-later
 # Copyright (C) 2021 Iñigo Martinez <inigomartinez@gmail.com>
 
-subdir('tests')
+if enable_tests
+  subdir('tests')
+endif
 
 service_conf = {
   'sbindir': mm_prefix / mm_sbindir,