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/linux-sandbox.sh

@@ -0,0 +1,75 @@
+source common.sh
+
+needLocalStore "the sandbox only runs on the builder side, so it makes no sense to test it with the daemon"
+
+clearStore
+
+requireSandboxSupport
+
+# Note: we need to bind-mount $SHELL into the chroot. Currently we
+# only support the case where $SHELL is in the Nix store, because
+# otherwise things get complicated (e.g. if it's in /bin, do we need
+# /lib as well?).
+if [[ ! $SHELL =~ /nix/store ]]; then skipTest "Shell is not from Nix store"; fi
+# An alias to automatically bind-mount the $SHELL on nix-build invocations
+nix-sandbox-build () { nix-build --no-out-link --sandbox-paths /nix/store "$@"; }
+
+chmod -R u+w $TEST_ROOT/store0 || true
+rm -rf $TEST_ROOT/store0
+
+export NIX_STORE_DIR=/my/store
+export NIX_REMOTE=$TEST_ROOT/store0
+
+outPath=$(nix-sandbox-build dependencies.nix)
+
+[[ $outPath =~ /my/store/.*-dependencies ]]
+
+nix path-info -r $outPath | grep input-2
+
+nix store ls -R -l $outPath | grep foobar
+
+nix store cat $outPath/foobar | grep FOOBAR
+
+# Test --check without hash rewriting.
+nix-sandbox-build dependencies.nix --check
+
+# Test that sandboxed builds with --check and -K can move .check directory to store
+nix-sandbox-build check.nix -A nondeterministic
+
+# `100 + 4` means non-determinstic, see doc/manual/src/command-ref/status-build-failure.md
+expectStderr 104 nix-sandbox-build check.nix -A nondeterministic --check -K > $TEST_ROOT/log
+grepQuietInverse 'error: renaming' $TEST_ROOT/log
+grepQuiet 'may not be deterministic' $TEST_ROOT/log
+
+# Test that sandboxed builds cannot write to /etc easily
+# `100` means build failure without extra info, see doc/manual/src/command-ref/status-build-failure.md
+expectStderr 100 nix-sandbox-build -E 'with import ./config.nix; mkDerivation { name = "etc-write"; buildCommand = "echo > /etc/test"; }' |
+    grepQuiet "/etc/test: Permission denied"
+
+
+## Test mounting of SSL certificates into the sandbox
+testCert () {
+    expectation=$1 # "missing" | "present"
+    mode=$2        # "normal" | "fixed-output"
+    certFile=$3    # a string that can be the path to a cert file
+    # `100` means build failure without extra info, see doc/manual/src/command-ref/status-build-failure.md
+    [ "$mode" == fixed-output ] && ret=1 || ret=100
+    expectStderr $ret nix-sandbox-build linux-sandbox-cert-test.nix --argstr mode "$mode" --option ssl-cert-file "$certFile" |
+        grepQuiet "CERT_${expectation}_IN_SANDBOX"
+}
+
+nocert=$TEST_ROOT/no-cert-file.pem
+cert=$TEST_ROOT/some-cert-file.pem
+echo -n "CERT_CONTENT" > $cert
+
+# No cert in sandbox when not a fixed-output derivation
+testCert missing normal       "$cert"
+
+# No cert in sandbox when ssl-cert-file is empty
+testCert missing fixed-output ""
+
+# No cert in sandbox when ssl-cert-file is a nonexistent file
+testCert missing fixed-output "$nocert"
+
+# Cert in sandbox when ssl-cert-file is set to an existing file
+testCert present fixed-output "$cert"