NIAP requires that the TLS KDF be tested by CAVP so this change moves
the PRF into crypto/fipsmodule/tls and adds a test harness for it. Like
the KAS tests, this is only triggered when “-niap” is passed to
run_cavp.go.

Change-Id: Iaa4973d915853c8e367e6106d829e44fcf1b4ce5
Reviewed-on: https://boringssl-review.googlesource.com/24666
Reviewed-by: Adam Langley <agl@google.com>