ATest: Fix test_flatten_test_infos flakeness.

Use deepcopy for the constant MODULE_BUILD_TARGETS when set to the
return value of mock to prevent the constants be modified after function
run.

Bug: 192827876
Test: atest atest_unittests --host
Change-Id: I5ee8036d151b05a7203046681a81328fa1bf6234
1 file changed