Keep the colleague's microbu-esp32c5 tree in this repository

obu-firmware builds against vanetza-idf from microbu-esp32c5/external, but
that tree was gitignored, so a clone of this repository could not build the
firmware it ships. It is now committed here as ordinary files in its own
folder, microbu-esp32c5/: the colleague's commit cf4b99f plus the V2X2MAP
bridge's signature verification (--trust) used on the bench. Nothing is
fetched from or pushed to the colleague's repository; this repository and
its remotes carry everything. The folder's own .gitignore keeps build output,
downloaded components and private key material out, as it did there; the
committed file set is identical to that repository's tracked files.

The ESP32-C5 is still flashed from obu-firmware/, which only takes
vanetza-idf from microbu-esp32c5/, so the two stay separate folders.
FLASHING.md says how to take a newer version of the colleague's tree (copy
it over the folder, rebuild, test, commit).

microbu-esp32c5/external/vanetza-idf/vanetza/dcc/tests/CMakeLists.txt

@@ -0,0 +1,15 @@
+include(UseGTest)
+configure_gtest_directory(LINK_LIBRARIES dcc)
+
+add_gtest(BurstBudget burst_budget.cpp)
+add_gtest(BurstyTransmitRateControl bursty_transmit_rate_control.cpp)
+add_gtest(ChannelLoad channel_load.cpp)
+add_gtest(FlowControl flow_control.cpp)
+add_gtest(FullyMeshedStateMachine fully_meshed_state_machine.cpp)
+add_gtest(GradualStateMachine gradual_state_machine.cpp)
+add_gtest(Limeric limeric.cpp)
+add_gtest(LimericBudget limeric_budget.cpp)
+add_gtest(Mapping mapping.cpp)
+add_gtest(SmoothingChannelProbeProcessor smoothing_channel_probe_processor.cpp)
+add_gtest(StateMachineBudget state_machine_budget.cpp)
+