aconfig: fix broken test 'aconfig.test.cpp.test_mode:AconfigTest#ResetFlagValue'

The aconfig.test.cpp.test_mode test cases modify the local flag
overrides, but was not resetting the values between tests. This meant
that

  $ atest 'aconfig.test.cpp.test_mode:AconfigTest#ResetFlagValue'

would succeed, but

  $ atest aconfig.test.cpp.test_mode

would fail. Fix this by calling reset_flags() as part of the per-test
setup.

Bug: N/A
Test: atest aconfig.test.cpp.test_mode
Change-Id: Ic8cffff0deb279b7ae103c05642c67230afc040f