Add PTF tests for Tutorial Exercise # 2

[aqn96]

Addresses part of #103 issue (exercise 2 tests)
Adds automated PTF tests for the basic_tunnel exercise that run against solution/basic_tunnel.p4.
Included:
ptf/basic_tunnel.py — Seven tests

• Ipv4DropOnMissTest — verifies plain IPv4 packets are dropped with no LPM table entries

• Ipv4ForwardTest — verifies IPv4 forwarding with correct MAC rewrite and TTL decrement

• TunnelForwardTest — verifies tunneled packets are forwarded correctly by dst_id

• TunnelDropOnMissTest — verifies tunneled packets are dropped with no tunnel table entry

• MixedTrafficTest — verifies IPv4 and tunnel traffic are handled independently

• TunnelUnknownProtoTest — verifies tunnel forwarding is based solely on dst_id regardless of proto_id

• TtlBoundaryTest — verifies TTL decrements correctly at boundary value

• runptf.sh — compiles solution, starts simple_switch_grpc, runs PTF tests, cleans up

• Makefile — adds make test target

• .gitignore — excludes build artifacts and log files

• test-exercises.yml — refactored CI to use matrix strategy, added concurrency and timeout. Used this as an example: https://github.com/p4lang/p4-dpdk-target/blob/main/.github/workflows/build_p4sde_ci.yml

Cleanup:

• Removed mention of make test from exercises/basic/README.md as per @jafingerhut suggestion that tests are intended for maintainers/contributors only, not students

Test Result:

[Dscano]

I would keep the Automated Tests section. It may not be the most interesting part from a learning perspective, but it is still valuable because it shows that the program is covered by tests.

Could you add a short descriptive sentence for this section? For example, you could explain that these automated tests are intended to support CI/CD, etc.

[Dscano]

@aqn96 overall, it looks good to me.