Rename the pixel conversion performance test

We want to have other tests related to pixel conversion, so avoid
possible naming conflicts.
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index fd47c15..cce2f1e 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -2,5 +2,5 @@
 
 add_library(test_util STATIC util.cxx)
 
-add_executable(pixelconv pixelconv.cxx)
-target_link_libraries(pixelconv test_util rfb)
+add_executable(convperf convperf.cxx)
+target_link_libraries(convperf test_util rfb)