libcamera: ProcessManager: make ProcessManager lifetime explicitly managed

If any Process instances are destroyed after the ProcessManager is
destroyed, then a segfault will occur.

Fix this by making the lifetime of the ProcessManager explicit, and make
the CameraManager construct and deconstruct (automatically, via a member
variable) the ProcessManager.

Update the tests accordingly.

Signed-off-by: Paul Elder <paul.elder@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>

src/libcamera/camera_manager.cpp

@@ -18,6 +18,7 @@
 #include "libcamera/internal/ipa_manager.h"
 #include "libcamera/internal/log.h"
 #include "libcamera/internal/pipeline_handler.h"
+#include "libcamera/internal/process.h"
 #include "libcamera/internal/thread.h"
 #include "libcamera/internal/utils.h"
 
@@ -67,6 +68,7 @@ private:
 	std::unique_ptr<DeviceEnumerator> enumerator_;
 
 	IPAManager ipaManager_;
+	ProcessManager processManager_;
 };
 
 CameraManager::Private::Private(CameraManager *cm)