tools/shelltest-runner: update agent shell test binary to agent/pingpong according to go/vcl/6065.
tools/shelltest-runner: update agent shell test binary to agent/pingpong according to go/vcl/6065.

Change-Id: I84082393d936ca1d6c465b67c96faab5b951cc48

vanadium/go.devtools,vanadium/go.devtools,vanadium/go.devtools