tests: Add env var tests

This adds some e2e tests for environment variables set in the
config.json. These were based on tests that failed on docker CI[1][2] after
the refactor on 06f1e0765 ("libct: speedup process.Env handling") and
some bugs that I had along the way trying to fix it.

These tests pass with runc 1.2 too.

[1]: 843e51459f/integration-cli/docker_cli_run_test.go (L822-L843)
[2]: 843e51459f/integration-cli/docker_cli_links_test.go (L197-L204)

Signed-off-by: Rodrigo Campos <rodrigoca@microsoft.com>
(cherry picked from commit 19c6515471)

tests/integration/env.bats

@@ -0,0 +1,97 @@
+#!/usr/bin/env bats
+# shellcheck disable=SC2016
+# This disables the check for shell variables inside single quotes
+# We do that all the time in this file, as we are testing env vars.
+
+load helpers
+
+function setup() {
+	setup_busybox
+}
+
+function teardown() {
+	teardown_bundle
+}
+
+# Several of these tests are inspired on regressions caught by Docker, besides other tests that
+# check the behavior we already had in runc:
+# https://github.com/moby/moby/blob/843e51459f14ebc964d349eba1013dc8a3e9d52e/integration-cli/docker_cli_links_test.go#L197-L204
+# https://github.com/moby/moby/blob/843e51459f14ebc964d349eba1013dc8a3e9d52e/integration-cli/docker_cli_run_test.go#L822-L843
+#
+
+@test "non-empty HOME env is used" {
+	update_config ' .process.env += ["HOME=/override"]'
+	update_config ' .process.args += ["-c", "echo $HOME"]'
+
+	runc run test_busybox
+	[ "$status" -eq 0 ]
+	[[ "${lines[0]}" == '/override' ]]
+}
+
+@test "empty HOME env var is overridden" {
+	update_config ' .process.env += ["HOME="]'
+	update_config ' .process.args += ["-c", "echo $HOME"]'
+
+	runc run test_busybox
+	[ "$status" -eq 0 ]
+	[[ "${lines[0]}" == '/root' ]]
+}
+
+@test "empty HOME env var is overridden with multiple overrides" {
+	update_config ' .process.env += ["HOME=/override", "HOME="]'
+	update_config ' .process.args += ["-c", "echo $HOME"]'
+
+	runc run test_busybox
+	[ "$status" -eq 0 ]
+	[[ "${lines[0]}" == '/root' ]]
+}
+
+@test "env var HOME is set only once" {
+	# env will show if an env var is set multiple times.
+	update_config ' .process.args = ["env"]'
+	update_config ' .process.env = ["HOME=", "PATH=/usr/bin:/bin"]'
+
+	runc run test_busybox
+	[ "$status" -eq 0 ]
+
+	# There should be 2 words/env-vars: HOME and PATH.
+	[ "$(wc -w <<<"$output")" -eq 2 ]
+}
+
+@test "env var override is set only once" {
+	# env will show if an env var is set multiple times.
+	update_config ' .process.args = ["env"]'
+	update_config ' .process.env = ["ONE=two", "ONE=", "PATH=/usr/bin:/bin"]'
+
+	runc run test_busybox
+	[ "$status" -eq 0 ]
+
+	# There should be 3 words/env-vars: ONE, PATH and HOME.
+	[ "$(wc -w <<<"$output")" -eq 3 ]
+}
+
+@test "env var override" {
+	update_config ' .process.env += ["ONE=two", "ONE=three"]'
+	update_config ' .process.args += ["-c", "echo ONE=\"$ONE\""]'
+
+	runc run test_busybox
+	[ "$status" -eq 0 ]
+	[[ "${lines[0]}" == "ONE=three" ]]
+}
+
+@test "env var with new-line is honored" {
+	update_config ' .process.env = ["NEW_LINE_ENV=\n", "PATH=/usr/bin:/bin"]'
+	update_config ' .process.args = ["env"]'
+
+	runc run test_busybox
+	[ "$status" -eq 0 ]
+
+	# There should be 4 lines
+	# NEW_LINE is a \n and when printed, it takes another line:
+	# 1. HOME=...
+	# 2. PATH=...
+	# 3. NEW_LINE_ENV=
+	# 4.
+	#
+	[ "$(wc -l <<<"$output")" -eq 4 ]
+}