Put functional tests in tests/functional

I think it is bad for these reasons when `tests/` contains a mix of
functional and integration tests

 - Concepts is harder to understand, the documentation makes a good
   unit vs functional vs integration distinction, but when the
   integration tests are just two subdirs within `tests/` this is not
   clear.

 - Source filtering in the `flake.nix` is more complex. We need to
   filter out some of the dirs from `tests/`, rather than simply pick
   the dirs we want and take all of them. This is a good sign the
   structure of what we are trying to do is not matching the structure
   of the files.

With this change we have a clean:
```shell-session
$ git show 'HEAD:tests'
tree HEAD:tests

functional/
installer/
nixos/
```

tests/functional/test-libstoreconsumer/README.md

@@ -0,0 +1,6 @@
+
+A very simple C++ consumer of the libstore library.
+
+  - Keep it simple. Library consumers expect something simple.
+  - No build hook, or any other reinvocations.
+  - No more global state than necessary.

tests/functional/test-libstoreconsumer/local.mk

@@ -0,0 +1,15 @@
+programs += test-libstoreconsumer
+
+test-libstoreconsumer_DIR := $(d)
+
+# do not install
+test-libstoreconsumer_INSTALL_DIR :=
+
+test-libstoreconsumer_SOURCES := \
+  $(wildcard $(d)/*.cc) \
+
+test-libstoreconsumer_CXXFLAGS += -I src/libutil -I src/libstore
+
+test-libstoreconsumer_LIBS = libstore libutil
+
+test-libstoreconsumer_LDFLAGS = -pthread $(SODIUM_LIBS) $(EDITLINE_LIBS) $(BOOST_LDFLAGS) $(LOWDOWN_LIBS)

tests/functional/test-libstoreconsumer/main.cc

@@ -0,0 +1,45 @@
+#include "globals.hh"
+#include "store-api.hh"
+#include "build-result.hh"
+#include <iostream>
+
+using namespace nix;
+
+int main (int argc, char **argv)
+{
+    try {
+        if (argc != 2) {
+            std::cerr << "Usage: " << argv[0] << " store/path/to/something.drv\n";
+            return 1;
+        }
+
+        std::string drvPath = argv[1];
+
+        initLibStore();
+
+        auto store = nix::openStore();
+
+        // build the derivation
+
+        std::vector<DerivedPath> paths {
+            DerivedPath::Built {
+                .drvPath = makeConstantStorePathRef(store->parseStorePath(drvPath)),
+                .outputs = OutputsSpec::Names{"out"}
+            }
+        };
+
+        const auto results = store->buildPathsWithResults(paths, bmNormal, store);
+
+        for (const auto & result : results) {
+            for (const auto & [outputName, realisation] : result.builtOutputs) {
+                std::cout << store->printStorePath(realisation.outPath) << "\n";
+            }
+        }
+
+        return 0;
+
+    } catch (const std::exception & e) {
+        std::cerr << "Error: " << e.what() << "\n";
+        return 1;
+    }
+}