snapshot/test: Add delay to all the CreateLogicalAndSnapshotPartitions

In tests CreateLogicalAndSnapshotPartitions may give flaky results
because not synchronous.
Add a bounded delay that prevents this behavior defined by a single
constexpr.

Bug: none
Test: libsnapshot_test
Change-Id: Iad3c24b1d3e82e3bd695d6dbbc8af8eab75bee32
Signed-off-by: Alessio Balsini <balsini@google.com>