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/fetchurl.sh

@@ -0,0 +1,80 @@
+source common.sh
+
+clearStore
+
+# Test fetching a flat file.
+hash=$(nix-hash --flat --type sha256 ./fetchurl.sh)
+
+outPath=$(nix-build -vvvvv --expr 'import <nix/fetchurl.nix>' --argstr url file://$(pwd)/fetchurl.sh --argstr sha256 $hash --no-out-link)
+
+cmp $outPath fetchurl.sh
+
+# Do not re-fetch paths already present.
+outPath2=$(nix-build -vvvvv --expr 'import <nix/fetchurl.nix>' --argstr url file:///does-not-exist/must-remain-unused/fetchurl.sh --argstr sha256 $hash --no-out-link)
+test "$outPath" == "$outPath2"
+
+# Now using a base-64 hash.
+clearStore
+
+hash=$(nix hash file --type sha512 --base64 ./fetchurl.sh)
+
+outPath=$(nix-build -vvvvv --expr 'import <nix/fetchurl.nix>' --argstr url file://$(pwd)/fetchurl.sh --argstr sha512 $hash --no-out-link)
+
+cmp $outPath fetchurl.sh
+
+# Now using an SRI hash.
+clearStore
+
+hash=$(nix hash file ./fetchurl.sh)
+
+[[ $hash =~ ^sha256- ]]
+
+outPath=$(nix-build -vvvvv --expr 'import <nix/fetchurl.nix>' --argstr url file://$(pwd)/fetchurl.sh --argstr hash $hash --no-out-link)
+
+cmp $outPath fetchurl.sh
+
+# Test that we can substitute from a different store dir.
+clearStore
+
+other_store=file://$TEST_ROOT/other_store?store=/fnord/store
+
+hash=$(nix hash file --type sha256 --base16 ./fetchurl.sh)
+
+storePath=$(nix --store $other_store store add-file ./fetchurl.sh)
+
+outPath=$(nix-build -vvvvv --expr 'import <nix/fetchurl.nix>' --argstr url file:///no-such-dir/fetchurl.sh --argstr sha256 $hash --no-out-link --substituters $other_store)
+
+# Test hashed mirrors with an SRI hash.
+nix-build -vvvvv --expr 'import <nix/fetchurl.nix>' --argstr url file:///no-such-dir/fetchurl.sh --argstr hash $(nix hash to-sri --type sha256 $hash) \
+          --no-out-link --substituters $other_store
+
+# Test unpacking a NAR.
+rm -rf $TEST_ROOT/archive
+mkdir -p $TEST_ROOT/archive
+cp ./fetchurl.sh $TEST_ROOT/archive
+chmod +x $TEST_ROOT/archive/fetchurl.sh
+ln -s foo $TEST_ROOT/archive/symlink
+nar=$TEST_ROOT/archive.nar
+nix-store --dump $TEST_ROOT/archive > $nar
+
+hash=$(nix-hash --flat --type sha256 $nar)
+
+outPath=$(nix-build -vvvvv --expr 'import <nix/fetchurl.nix>' --argstr url file://$nar --argstr sha256 $hash \
+          --arg unpack true --argstr name xyzzy --no-out-link)
+
+echo $outPath | grepQuiet 'xyzzy'
+
+test -x $outPath/fetchurl.sh
+test -L $outPath/symlink
+
+nix-store --delete $outPath
+
+# Test unpacking a compressed NAR.
+narxz=$TEST_ROOT/archive.nar.xz
+rm -f $narxz
+xz --keep $nar
+outPath=$(nix-build -vvvvv --expr 'import <nix/fetchurl.nix>' --argstr url file://$narxz --argstr sha256 $hash \
+          --arg unpack true --argstr name xyzzy --no-out-link)
+
+test -x $outPath/fetchurl.sh
+test -L $outPath/symlink