tests: restore the initial generation before each test

When developing it can happen to break the nix-on-droid script. If so,
this fix will ensure that each test has a working nix-on-droid script
available.

tests/on-device/.run.sh

@@ -6,8 +6,11 @@
 set -ueo pipefail
 
 SCRIPT_DIR="$(dirname "$(readlink -f "$0")")"
+PROFILE_DIRECTORY="/nix/var/nix/profiles/nix-on-droid"
+
 SELF_TEST_DIR="$HOME/.cache/nix-on-droid-self-test"
 CONFIRMATION_FILE="$SELF_TEST_DIR/confirmation-granted"
+DEFAULT_ACTIVATE_SCRIPT="$SELF_TEST_DIR/default-activate"
 mkdir -p "$SELF_TEST_DIR"
 
 if [[ ! -e "$CONFIRMATION_FILE" ]]; then
@@ -21,6 +24,10 @@ if [[ ! -e "$CONFIRMATION_FILE" ]]; then
     touch "$CONFIRMATION_FILE"
 fi
 
+if [[ ! -e "$DEFAULT_ACTIVATE_SCRIPT" ]]; then
+    ln -sn "$(readlink -f "$PROFILE_DIRECTORY/activate")" "$DEFAULT_ACTIVATE_SCRIPT"
+fi
+
 if [[ ! -d ~/.config.bak ]]; then
     mv ~/.config ~/.config.bak
     cp -r ~/.config.bak ~/.config

tests/on-device/lib.bash

@@ -23,6 +23,9 @@ setup() {
 
     ON_DEVICE_TESTS_SETUP=1
   fi
+
+  # restore to pre-testing generation before the start of each test
+  $DEFAULT_ACTIVATE_SCRIPT
 }
 
 assert_command() {