tests: add variable to suppress TTS tests

TTS support can be gated on e.g. Gentoo's USE=speech, so we don't want
to run the basic selfcheck of speech support if this is disabled. We do
want to run the general Qt checks, though (and also do a selfcheck of
speech support if the USE=speech is enabled!) -- so add a sneaky
environment variable that we can use to conditionally skip running those
parts.

src/calibre/test_build.py

@@ -123,6 +123,7 @@ class BuildTest(unittest.TestCase):
         from speechd.client import SSIPClient
         del SSIPClient
 
+    @unittest.skipIf('SKIP_SPEECH_TESTS' in os.environ, 'Speech support is opted out')
     def test_piper(self):
         import subprocess
 
@@ -322,7 +323,7 @@ class BuildTest(unittest.TestCase):
     def test_qt(self):
         if is_sanitized:
             raise unittest.SkipTest('Skipping Qt build test as sanitizer is enabled')
-        from qt.core import QApplication, QFontDatabase, QImageReader, QLoggingCategory, QMediaDevices, QNetworkAccessManager, QSslSocket, QTextToSpeech, QTimer
+        from qt.core import QApplication, QFontDatabase, QImageReader, QLoggingCategory, QNetworkAccessManager, QSslSocket, QTimer
         QLoggingCategory.setFilterRules('''qt.webenginecontext.debug=true''')
         if hasattr(os, 'geteuid') and os.geteuid() == 0:
             # likely a container build, webengine cannot run as root with sandbox
@@ -356,12 +357,15 @@ class BuildTest(unittest.TestCase):
         try:
             ensure_app()
             self.assertGreaterEqual(len(QFontDatabase.families()), 5, 'The QPA headless plugin is not able to locate enough system fonts via fontconfig')
-            available_tts_engines = tuple(x for x in QTextToSpeech.availableEngines() if x != 'mock')
-            self.assertTrue(available_tts_engines)
 
-            QMediaDevices.audioOutputs()
+            if 'SKIP_SPEECH_TESTS' not in os.environ:
+                from qt.core import QMediaDevices, QTextToSpeech
+
+                available_tts_engines = tuple(x for x in QTextToSpeech.availableEngines() if x != 'mock')
+                self.assertTrue(available_tts_engines)
+
+                QMediaDevices.audioOutputs()
 
-            self.assertGreaterEqual
             from calibre.ebooks.oeb.transforms.rasterize import rasterize_svg
             img = rasterize_svg(as_qimage=True)
             self.assertFalse(img.isNull())

src/calibre/utils/run_tests.py

@@ -190,6 +190,8 @@ class TestImports(unittest.TestCase):
                 'calibre.utils.linux_trash', 'calibre.utils.open_with.linux',
                 'calibre.gui2.linux_file_dialogs',
             }
+        if 'SKIP_SPEECH_TESTS' in os.environ:
+            exclude_packages.add('calibre.gui2.tts')
         if not isbsd:
             exclude_modules.add('calibre.devices.usbms.hal')
         d = os.path.dirname