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FLASHING.md says how to take a newer version of the colleague's tree (copy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/common/tests/lru_cache.cpp

@@ -0,0 +1,82 @@
+#include <gtest/gtest.h>
+#include <vanetza/common/lru_cache.hpp>
+
+using namespace vanetza;
+
+class LruCacheTest : public ::testing::Test
+{
+protected:
+    void SetUp() override
+    {
+        generator_calls = 0;
+        last_key = 0;
+    }
+
+    std::function<int(int)> generator()
+    {
+        return [this](int key) {
+            ++generator_calls;
+            last_key = key;
+            return ~key;
+        };
+    }
+
+    unsigned generator_calls;
+    int last_key;
+};
+
+
+TEST_F(LruCacheTest, caching)
+{
+    LruCache<int, int> cache(generator(), 5);
+
+    // generate one new entry
+    EXPECT_EQ(~8, cache[8]);
+    EXPECT_EQ(1, generator_calls);
+
+    // refer to cached entry
+    EXPECT_EQ(~8, cache[8]);
+    EXPECT_EQ(1, generator_calls);
+
+    // further entry
+    EXPECT_EQ(~4, cache[4]);
+    EXPECT_EQ(2, generator_calls);
+
+    // first entry is still there
+    EXPECT_EQ(~8, cache[8]);
+    EXPECT_EQ(2, generator_calls);
+}
+
+TEST_F(LruCacheTest, modify)
+{
+    LruCache<int, int> cache(generator(), 5);
+
+    // modify new cache entry
+    cache[8] = 23;
+    EXPECT_EQ(23, cache[8]);
+
+    // modify existing entry
+    cache[8] = 42;
+    EXPECT_EQ(42, cache[8]);
+
+    EXPECT_EQ(1, generator_calls);
+}
+
+TEST_F(LruCacheTest, drop_lru)
+{
+    LruCache<int, int> cache(generator(), 3);
+    cache[1];
+    cache[2];
+    cache[3];
+    EXPECT_EQ(3, generator_calls);
+
+    EXPECT_EQ(~2, cache[2]); // LRU: 1, 3, 2
+    cache[4]; // drop 1, LRU: 3, 2, 4
+    cache[1]; // re-create entry, LRU: 2, 4, 1
+    EXPECT_EQ(5, generator_calls);
+
+    EXPECT_EQ(~2, cache[2]);
+    EXPECT_EQ(~4, cache[4]);
+    EXPECT_EQ(~1, cache[1]);
+    EXPECT_EQ(5, generator_calls);
+}