Add secondary test user (#1186)

## Description

Wire up code to fetch secondary user from config/env and also provide it to tests.

## Type of change

<!--- Please check the type of change your PR introduces: --->
- [x] 🌻 Feature
- [ ] 🐛 Bugfix
- [ ] 🗺️ Documentation
- [ ] 🤖 Test
- [x] 💻 CI/Deployment
- [ ] 🐹 Trivial/Minor

## Issue(s)

* closes #1185 

merge after to ensure automatic cleanup:
* #1193 

## Test Plan

<!-- How will this be tested prior to merging.-->
- [x] 💪 Manual
- [ ] ⚡ Unit test
- [ ] 💚 E2E

src/internal/tester/config.go

@@ -22,12 +22,14 @@ const (
 	// M365 config
 	TestCfgAzureTenantID   = "azure_tenantid"
 	TestCfgUserID          = "m365userid"
+	TestCfgSecondaryUserID = "secondarym365userid"
 	TestCfgAccountProvider = "account_provider"
 )
 
 // test specific env vars
 const (
 	EnvCorsoM365TestUserID          = "CORSO_M356_TEST_USER_ID"
+	EnvCorsoSecondaryM365TestUserID = "CORSO_SECONDARY_M356_TEST_USER_ID"
 	EnvCorsoTestConfigFilePath      = "CORSO_TEST_CONFIG_FILE"
 )
 
@@ -108,6 +110,13 @@ func readTestConfig() (map[string]string, error) {
 		vpr.GetString(TestCfgUserID),
 		"lidiah@8qzvrj.onmicrosoft.com",
 	)
+	fallbackTo(
+		testEnv,
+		TestCfgSecondaryUserID,
+		os.Getenv(EnvCorsoSecondaryM365TestUserID),
+		vpr.GetString(TestCfgSecondaryUserID),
+		"lynner@8qzvrj.onmicrosoft.com",
+	)
 
 	testEnv[EnvCorsoTestConfigFilePath] = os.Getenv(EnvCorsoTestConfigFilePath)
 	testConfig = testEnv

src/internal/tester/users.go

@@ -16,3 +16,15 @@ func M365UserID(t *testing.T) string {
 
 	return cfg[TestCfgUserID]
 }
+
+// SecondaryM365UserID returns an userID string representing the m365UserID
+// described by either the env var CORSO_SECONDARY_M356_TEST_USER_ID, the
+// corso_test.toml config file or the default value (in that order of priority).
+// The default is a last-attempt fallback that will only work on alcion's
+// testing org.
+func SecondaryM365UserID(t *testing.T) string {
+	cfg, err := readTestConfig()
+	require.NoError(t, err, "retrieving secondary m365 user id from test configuration")
+
+	return cfg[TestCfgSecondaryUserID]
+}