Use RFC5735/RFC5156 IP addresses in tests/docs

RFC5735 defines several ranges of special-use IPv4 addresses, including 3
intended for documation, examples and testing. Using these IP addresses avoids
copy and paste leaks resulting in errant traffic to the unfortunate assignees
of addresses such as 1.2.3.4. Likewise, RFC5156 defines similar ranges for IPv6
addresses.

This change modifies the test-cases, mock-providers and documentation to use
addresses from the TEST-NET ranges. For IPv4, CIDR semantics have been
preserved and the replacement IPs differ in the same levels of specificity
as the original IPs used. For IPv6, all addresses have been updated to
reside in the 2001:db8/32 range.

The EC2 DataFromInstanceMetadataHookTest still contains actual EC2 addresses,
as I'm unsure of the effect of a change here.

All tests pass, and this change contains no functional changes.