GpuMemTracerTest - switch mTestMap bpfmap init to move assignment

also remove use of .getMap() which I want to remove as it exposes fds and can leak
(isValid check in the next line is equivalent)

Bug: 236285127
Test: TreeHugger
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I4a8a3b918f25501ab0ab4aa74ce5d2a629ece463