libsnapshot: Temporarily disable flaky ImageManagerTest

This test was causing the failure of vts_libsnapshot_test.
Disable until a fix is found and tests are stable again.

Bug: 149738928
Test: atests vts_libsnapshot_test
Change-Id: Ibbe5cd72aad93d9ade4ac1542f08dec9cd926a3b
Signed-off-by: Alessio Balsini <balsini@google.com>