Migrate reinitialization test to Cucumber.

test/features/step_definitions/corner_cases.rb

@@ -0,0 +1,45 @@
+# frozen_string_literal: true
+
+Given('environment variable {env_name} is not set') do |name|
+  @name = name
+  @expect_intermediate_value = false
+  @command = <<~BASH
+    ( \\
+      env | grep -E "^#{name}="; \\
+      export -n #{name}; \\
+      env | grep -E "^#{name}="; \\
+  BASH
+end
+
+Given('environment variable {env_name} is set to {string}') do |name, new_value|
+  @name = name
+  @expect_intermediate_value = true
+  @command = <<~BASH
+    ( \\
+      env | grep -E "^#{name}="; \\
+      export #{name}=#{new_value}; \\
+      env | grep -E "^#{name}="; \\
+  BASH
+end
+
+When('a new Fish shell is launched') do
+  @command += <<~BASH
+      fish -lc "env | grep -E \\"^#{@name}=\\"" \\
+    ) \\
+  BASH
+
+  @response = run_bash_command(@command)
+end
+
+Then('environment variable {env_name} has the original value') do |name|
+  expect(name).to eq(@name) # otherwise the test doesn't make sense
+
+  if @expect_intermediate_value
+    expect(@response[:stdout].count).to eq(3)
+    expect(@response[:stdout][0]).to_not eq(@response[:stdout][1]) # destruction effective
+  else
+    expect(@response[:stdout].count).to eq(2)
+  end
+
+  expect(@response[:stdout][-1]).to eq(@response[:stdout][0]) # reinitialization effective
+end