test: Remove using namespace in header files

"using namespace" in a header file propagates the namespace to
the files including the header file. So it should be avoided.
This removes "using namespace" in header files in test.

Signed-off-by: Hirokazu Honda <hiroh@chromium.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

test/libtest/buffer_source.cpp

@@ -14,6 +14,8 @@
 
 #include "test.h"
 
+using namespace libcamera;
+
 BufferSource::BufferSource()
 {
 }