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/
```

(cherry picked from commit 68c81c7375)

tests/functional/bash-profile.sh

@@ -1,6 +1,6 @@
 source common.sh
 
-sed -e "s|@localstatedir@|$TEST_ROOT/profile-var|g" -e "s|@coreutils@|$coreutils|g" < ../scripts/nix-profile.sh.in > $TEST_ROOT/nix-profile.sh
+sed -e "s|@localstatedir@|$TEST_ROOT/profile-var|g" -e "s|@coreutils@|$coreutils|g" < ../../scripts/nix-profile.sh.in > $TEST_ROOT/nix-profile.sh
 
 user=$(whoami)
 rm -rf $TEST_HOME $TEST_ROOT/profile-var

tests/functional/build-remote-trustless.sh

@@ -6,7 +6,7 @@ unset NIX_STATE_DIR
 
 remoteDir=$TEST_ROOT/remote
 
-# Note: ssh{-ng}://localhost bypasses ssh. See tests/build-remote.sh for
+# Note: ssh{-ng}://localhost bypasses ssh. See tests/functional/build-remote.sh for
 # more details.
 nix-build $file -o $TEST_ROOT/result --max-jobs 0 \
   --arg busybox $busybox \

tests/functional/ca/local.mk

@@ -25,4 +25,4 @@ clean-files += \
   $(d)/config.nix
 
 test-deps += \
-  tests/ca/config.nix
+  tests/functional/ca/config.nix

tests/functional/common/vars-and-functions.sh.in

@@ -6,7 +6,7 @@ COMMON_VARS_AND_FUNCTIONS_SH_SOURCED=1
 
 export PS4='+(${BASH_SOURCE[0]-$0}:$LINENO) '
 
-export TEST_ROOT=$(realpath ${TMPDIR:-/tmp}/nix-test)/${TEST_NAME:-default}
+export TEST_ROOT=$(realpath ${TMPDIR:-/tmp}/nix-test)/${TEST_NAME:-default/tests\/functional//}
 export NIX_STORE_DIR
 if ! NIX_STORE_DIR=$(readlink -f $TEST_ROOT/store 2> /dev/null); then
     # Maybe the build directory is symlinked.

tests/functional/dyn-drv/local.mk

@@ -12,4 +12,4 @@ clean-files += \
   $(d)/config.nix
 
 test-deps += \
-  tests/dyn-drv/config.nix
+  tests/functional/dyn-drv/config.nix