Add test cases for `addEnv` which is a bit complicated function

Signed-off-by: Yuichiro Kaneko <spiketeika@gmail.com>

frontend/dockerfile/dockerfile2llb/convert_test.go

@@ -62,3 +62,53 @@ COPY --from=0 f2 /
 	_, _, err = Dockerfile2LLB(appcontext.Context(), []byte(df), ConvertOpt{})
 	assert.Error(t, err)
 }
+
+func TestAddEnv(t *testing.T) {
+	// k exists in env as key
+	// override = false
+	env := []string{"key1=val1", "key2=val2"}
+	result := addEnv(env, "key1", "value1", false)
+	assert.Equal(t, []string{"key1=val1", "key2=val2"}, result)
+
+	// k exists in env as key
+	// override = true
+	env = []string{"key1=val1", "key2=val2"}
+	result = addEnv(env, "key1", "value1", true)
+	assert.Equal(t, []string{"key1=value1", "key2=val2"}, result)
+
+	// k does not exist in env as key
+	// override = false
+	env = []string{"key1=val1", "key2=val2"}
+	result = addEnv(env, "key3", "val3", false)
+	assert.Equal(t, []string{"key1=val1", "key2=val2", "key3=val3"}, result)
+
+	// k does not exist in env as key
+	// override = true
+	env = []string{"key1=val1", "key2=val2"}
+	result = addEnv(env, "key3", "val3", true)
+	assert.Equal(t, []string{"key1=val1", "key2=val2", "key3=val3"}, result)
+
+	// env has same keys
+	// override = false
+	env = []string{"key1=val1", "key1=val2"}
+	result = addEnv(env, "key1", "value1", false)
+	assert.Equal(t, []string{"key1=val1", "key1=val2"}, result)
+
+	// env has same keys
+	// override = true
+	env = []string{"key1=val1", "key1=val2"}
+	result = addEnv(env, "key1", "value1", true)
+	assert.Equal(t, []string{"key1=value1", "key1=val2"}, result)
+
+	// k matches with key only string in env
+	// override = false
+	env = []string{"key1=val1", "key2=val2", "key3"}
+	result = addEnv(env, "key3", "val3", false)
+	assert.Equal(t, []string{"key1=val1", "key2=val2", "key3"}, result)
+
+	// k matches with key only string in env
+	// override = true
+	env = []string{"key1=val1", "key2=val2", "key3"}
+	result = addEnv(env, "key3", "val3", true)
+	assert.Equal(t, []string{"key1=val1", "key2=val2", "key3=val3"}, result)
+}