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>

test/process/process_test.cpp

@@ -87,6 +87,8 @@ private:
 		exitCode_ = exitCode;
 	}
 
+	ProcessManager processManager_;
+
 	Process proc_;
 	enum Process::ExitStatus exitStatus_;
 	int exitCode_;