Add CU-UP load tester and documentation

This is a simple CU-UP load tester. It sets up a configurable number of UEs in the
CU-UP, then sends GTP traffic in downlink/uplink in the N3-F1/F1-N3
direction at the CU-UP. It then compares the sent and received data.

Additional parameters, such as amount of traffic, test length, packet
size, and IP addresses/ports are configurable.

To run:

    mkdir build && cd build && cmake .. -GNinja && ninja nr-cuup nr-cuup-load-test params_libconfig
    ./tests/nr-cuup/nr-cuup-load-test
    ./nr-cuup -O ../tests/nr-cuup/load-test.conf

The tester should be able to detect lost and reordered packets.

Example lost: Receive order 2,3,5,6 (lost packet 4)
=> on packet 2, expected 2, next=expected+1=3
=> on packet 3, expected 3, next=expected+1=4
=> on packet 5, expected 4, diff=5-4=1 (lost 1), next=payload+1=6
=> on packet 6, expected 6
=> 1 lost

Example reordered: Receiver order 2,5,3,6 (lost packet 4)
=> on packet 2, expected 2, next=expected+1=3
=> on packet 5, expected 3, diff=5-3=2 (lost 2), next=payload+1=6
=> on packet 3, expected 6, (reduce lost by 1 to 1), next=expected=6
=> on packet 6, expected 6
=> 1 lost

Example reordered: Receiver order 2,5,3,4,6 (no losses)

doc/README.md

@@ -130,6 +130,8 @@ The other SDRs (AW2S, LimeSDR, ...) have no READMEs.
 # Testing
 
 - [UnitTests.md](./UnitTests.md) explains the unit testing setup
+- Component tests are under `tests/`. Currently, there is a simple CU-UP
+  tester, see the corresponding [README.md](../tests/nr-cuup/README.md).
 - [TESTBenches.md](./TESTBenches.md) lists the CI setup and links to pipelines
 
 # Developer tools