tests: define ENABLE_JUNIT_XML to 1 or 0

Code is using the form
  #if ENABLE_JUNIT_XML
which is fine until we start using -Wundef. I think the existing code
would fail or at least warn if you disabled test-junit-xml with -Wundef.

Make sure ENABLE_JUNIT_XML is always defined so that -Wundef can be
added to build flags.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>

tests/meson.build

@@ -73,13 +73,13 @@ exe_plugin_test = shared_library(
 )
 
 deps_zuc = [ dep_libshared ]
+config_h.set10('ENABLE_JUNIT_XML', get_option('test-junit-xml'))
 if get_option('test-junit-xml')
 	d = dependency('libxml-2.0', version: '>= 2.6', required: false)
 	if not d.found()
 		error('JUnit XML support requires libxml-2.0 >= 2.6 which was not found. Or, you can use \'-Dtest-junit-xml=false\'.')
 	endif
 	deps_zuc += d
-	config_h.set('ENABLE_JUNIT_XML', '1')
 endif
 
 lib_zuc = static_library(