tests/test-dbus-fixture.h: Turn TestDBusFixture::TestDBusFixture into an explicitly declared constructor.

 Resolves issues like these:

 ```
 In file included from /rootdir/tests/test-actions.cpp:22:
 In file included from /rootdir/tests/state-fixture.h:23:
 In file included from /rootdir/tests/test-dbus-fixture.h:23:
 In file included from /rootdir/tests/glib-fixture.h:29:
 In file included from /usr/include/gtest/gtest.h:62:
 /usr/include/gtest/internal/gtest-internal.h:472:44: error: call to implicitly-deleted default constructor of 'ActionsFixture_DesktopOpenCalendarApp_Test'
   Test* CreateTest() override { return new TestClass; }
                                            ^
 /rootdir/tests/test-actions.cpp:188:1: note: in instantiation of member function 'testing::internal::TestFactoryImpl<ActionsFixture_DesktopOpenCalendarApp_Test>::CreateTest' requested here
 TEST_F(ActionsFixture, DesktopOpenCalendarApp)
 ^
 /usr/include/gtest/gtest.h:2381:3: note: expanded from macro 'TEST_F'
   GTEST_TEST_(test_fixture, test_name, test_fixture, \
   ^
 /usr/include/gtest/internal/gtest-internal.h:1565:15: note: expanded from macro 'GTEST_TEST_'
           new ::testing::internal::TestFactoryImpl<GTEST_TEST_CLASS_NAME_(    \
               ^
 /rootdir/tests/test-actions.cpp:188:1: note: explicitly defaulted function was implicitly deleted here
 /usr/include/gtest/gtest.h:2381:3: note: expanded from macro 'TEST_F'
   GTEST_TEST_(test_fixture, test_name, test_fixture, \
   ^
 /usr/include/gtest/internal/gtest-internal.h:1544:5: note: expanded from macro 'GTEST_TEST_'
     GTEST_TEST_CLASS_NAME_(test_suite_name, test_name)() = default;           \
     ^
 /usr/include/gtest/internal/gtest-internal.h:1533:3: note: expanded from macro 'GTEST_TEST_CLASS_NAME_'
   test_suite_name##_##test_name##_Test
   ^
 <scratch space>:101:1: note: expanded from here
 ActionsFixture_DesktopOpenCalendarApp_Test
 ^
 /rootdir/tests/test-actions.cpp:188:1: note: default constructor of 'ActionsFixture_DesktopOpenCalendarApp_Test' is implicitly deleted because base class 'ActionsFixture' has a deleted default constructor
 /usr/include/gtest/gtest.h:2381:3: note: expanded from macro 'TEST_F'
   GTEST_TEST_(test_fixture, test_name, test_fixture, \
   ^
 /usr/include/gtest/internal/gtest-internal.h:1542:9: note: expanded from macro 'GTEST_TEST_'
       : public parent_class {                                                 \
         ^
 /rootdir/tests/test-actions.cpp:26:23: note: default constructor of 'ActionsFixture' is implicitly deleted because base class 'ayatana::indicator::datetime::StateFixture' has a deleted default constructor
 class ActionsFixture: public StateFixture
                       ^
 /rootdir/tests/state-fixture.h:36:21: note: default constructor of 'StateFixture' is implicitly deleted because base class 'TestDBusFixture' has a deleted default constructor
 class StateFixture: public TestDBusFixture
                     ^
 /rootdir/tests/test-dbus-fixture.h:33:5: note: explicitly defaulted function was implicitly deleted here
     TestDBusFixture() =default;
     ^
 /rootdir/tests/test-dbus-fixture.h:70:36: note: default constructor of 'TestDBusFixture' is implicitly deleted because field 'service_dirs' of const-qualified type 'const std::vector<std::string>' (aka 'const vector<basic_string<char>>') would not be initialized
     const std::vector<std::string> service_dirs;
                                    ^
 ```

1 files changed, 1 insertions, 1 deletions

diff --git a/tests/test-dbus-fixture.h b/tests/test-dbus-fixture.h
index a894f11..0f19b50 100644
--- a/tests/test-dbus-fixture.h
+++ b/tests/test-dbus-fixture.h
@@ -30,7 +30,7 @@ class TestDBusFixture: public GlibFixture
 {
   public:
 
-    TestDBusFixture() =default;
+    explicit TestDBusFixture() {};
     virtual ~TestDBusFixture() =default;
 
     explicit TestDBusFixture(const std::vector<std::string>& service_dirs_in): service_dirs(service_dirs_in) {}