Regression testing: Use mainline

Author: ax3l

run_test.sh

@@ -59,7 +59,7 @@ cd picsar && git checkout --detach a78be127f66adc1558f527edc8964e37e3a055ff && c
 git clone --depth 1 https://github.com/ECP-WarpX/warpx-data.git
 
 # Clone the AMReX regression test utility
-git clone https://github.com/ECP-WarpX/regression_testing.git
+git clone https://github.com/AMReX-Codes/regression_testing.git
 
 # Prepare regression tests
 mkdir -p rt-WarpX/WarpX-benchmarks