meson: separate meson.build for tests

Separate build file reduces indentation and makes things more readable.

Tested: Unit Tests Pass

```
Running all tests
1/4 test_systemd_parser                   OK              0.03s
2/4 test_systemd_signal                   OK              0.02s
3/4 test_scheduled_host_transition        OK              0.01s
4/4 test_hypervisor_state                 OK              0.01s

Ok:                4
Fail:              0
```

Change-Id: I369d00698eddad28834aeebc5455fe9387019f9d
Signed-off-by: Alexander Hansen <alexander.hansen@9elements.com>
diff --git a/meson.build b/meson.build
index 65dc445..77c44b8 100644
--- a/meson.build
+++ b/meson.build
@@ -312,68 +312,5 @@
         endif
     endif
 
-    test(
-        'test_systemd_parser',
-        executable(
-            'test_systemd_parser',
-            './test/systemd_parser.cpp',
-            'systemd_target_parser.cpp',
-            dependencies: [gtest, nlohmann_json_dep],
-            implicit_include_directories: true,
-            include_directories: '../',
-        ),
-    )
-
-    test(
-        'test_systemd_signal',
-        executable(
-            'test_systemd_signal',
-            './test/systemd_signal.cpp',
-            'systemd_target_signal.cpp',
-            'utils.cpp',
-            dependencies: [
-                gtest,
-                libgpiod,
-                nlohmann_json_dep,
-                phosphorlogging,
-                sdbusplus,
-                sdeventplus,
-            ],
-            implicit_include_directories: true,
-            include_directories: '../',
-        ),
-    )
-
-    test(
-        'test_scheduled_host_transition',
-        executable(
-            'test_scheduled_host_transition',
-            './test/test_scheduled_host_transition.cpp',
-            'scheduled_host_transition.cpp',
-            'utils.cpp',
-            dependencies: [
-                cereal,
-                gmock,
-                gtest,
-                libgpiod,
-                phosphorlogging,
-                sdbusplus,
-                sdeventplus,
-            ],
-            implicit_include_directories: true,
-            include_directories: '../',
-        ),
-    )
-
-    test(
-        'test_hypervisor_state',
-        executable(
-            'test_hypervisor_state',
-            './test/hypervisor_state.cpp',
-            'hypervisor_state_manager.cpp',
-            dependencies: [gtest, phosphorlogging, sdbusplus, sdeventplus],
-            implicit_include_directories: true,
-            include_directories: '../',
-        ),
-    )
+    subdir('test')
 endif