tests: add assertions only relevant for the specific test case

tests/on-device/config-default.bats

@@ -4,4 +4,7 @@ load lib
 
 @test 'default config can be switched into' {
   switch_to_default_config
+
+  assert_command nix-on-droid nix-shell bash vi find
+  assert_no_command dash xonsh zsh
 }

tests/on-device/config-flake-h-m.bats

@@ -3,8 +3,7 @@
 load lib
 
 @test 'flake + h-m + #134 overlays case work' {
-  # start from a known baseline
-  switch_to_default_config
+  # assertions to verify initial state is as expected
   assert_command vi
   assert_no_command dash zsh
 

tests/on-device/config-flake.bats

@@ -3,11 +3,11 @@
 load lib
 
 @test 'flake example works' {
-  # start from a known baseline
-  switch_to_default_config
+  # assertions to verify initial state is as expected
   assert_command vi
-  assert_no_command dash unzip
+  assert_no_command unzip
 
+  # set up / build / activate the configuration
   nix-shell -p gnused --run \
     "sed \
          -e s/vim/nano/ \
@@ -39,5 +39,5 @@ load lib
   rm -f ~/.config/nix/nix.conf ~/.config/nixpkgs/flake.nix
   switch_to_default_config
   assert_command vi
-  assert_no_command dash unzip
+  assert_no_command unzip
 }

tests/on-device/config-h-m.bats

@@ -2,13 +2,17 @@
 
 load lib
 
+teardown() {
+  rm -f ~/.config/example
+}
+
 @test 'using home-manager works' {
-  # start from a known baseline
-  switch_to_default_config
+  # assertions to verify initial state is as expected
   assert_command vi
   assert_no_command dash
-  ! [[ -e ~/.config/example ]]
+  [[ ! -e ~/.config/example ]]
 
+  # set up / build / activate the configuration
   nix-channel --add https://github.com/rycee/home-manager/archive/release-21.11.tar.gz home-manager
   nix-channel --update
   cp "$ON_DEVICE_TESTS_DIR/config-h-m.nix" ~/.config/nixpkgs/nix-on-droid.nix
@@ -30,5 +34,11 @@ load lib
   [[ "$output" == success ]]
   [[ "$status" == 42 ]]
 
+  # check that reverting works too
   switch_to_default_config
+  assert_command vi
+  assert_no_command unzip
+
+  # file will be still present because home-manager needs to be set up to remove old links
+  [[ -e ~/.config/example ]]
 }

tests/on-device/lib.bash

@@ -26,6 +26,9 @@ setup() {
 
   # restore to pre-testing generation before the start of each test
   $DEFAULT_ACTIVATE_SCRIPT
+
+  # build and activate the version of nix-on-droid that is subject to test
+  switch_to_default_config
 }
 
 assert_command() {
@@ -47,7 +50,4 @@ switch_to_default_config() {
   cat "$CHANNEL_DIR/modules/environment/login/nix-on-droid.nix.default" \
     > ~/.config/nixpkgs/nix-on-droid.nix
   nix-on-droid switch
-
-  assert_command nix-on-droid nix-shell bash vi find
-  assert_no_command dash xonsh zsh
 }