gn2bp: Change stem for test targets

* Stem should be the name of the target under GN build system as this is the one expected in the source code
as seen here https://source.chromium.org/chromium/chromium/src/+/main:components/cronet/android/test/javatests/src/org/chromium/net/CronetTestRule.java;l=244?q=loadLibrary%5C(%5C%22cronet_tests&ss=chromium%2Fchromium%2Fsrc

Test: None
Change-Id: I758b9b265d0187c93b4fc8ab48491cc5f7a5a750