test: integration tests for env manager

[DenserMeerkat]

PR Description

Integration tests for the feature environment manager.

Related Issues

• Related Issue #445
• Closes #445

Checklist

• [x] I have gone through the contributing guide
• [x] I have updated my branch and synced it with project main branch before making this PR
• [x] I have run the tests (flutter test) and all tests are passing

Added/updated tests?

We encourage you to add relevant test cases.

• [x] Yes
• [ ] No, and this is why: please replace this line with details on why tests have not been included

[ashitaprasad]

A test failed for this PR:

/Documents/GitHub/apidash/test/providers/ui_providers_test.dart: Testing navRailIndexStateProvider UI should update correctly when navRailIndexStateProvider changes
══╡ EXCEPTION CAUGHT BY FLUTTER TEST FRAMEWORK ╞════════════════════════════════════════════════════
The following TestFailure was thrown running a test:
Expected: exactly one matching candidate
  Actual: _IconWidgetFinder:<Found 0 widgets with icon "IconData(U+0F153)": []>
   Which: means none were found but one was expected

When the exception was thrown, this was the stack:
#4      main.<anonymous closure>.<anonymous closure> (file:///Documents/GitHub/apidash/test/providers/ui_providers_test.dart:220:7)
<asynchronous suspension>
#5      testWidgets.<anonymous closure>.<anonymous closure> (package:flutter_test/src/widget_tester.dart:183:15)
<asynchronous suspension>
#6      TestWidgetsFlutterBinding._runTestBody (package:flutter_test/src/binding.dart:1017:5)
<asynchronous suspension>
<asynchronous suspension>
(elided one frame from package:stack_trace)

This was caught by the test expectation on the following line:
  file:///Documents/GitHub/apidash/test/providers/ui_providers_test.dart line 220
The test description was:
  UI should update correctly when navRailIndexStateProvider changes
════════════════════════════════════════════════════════════════════════════════════════════════════
00:18 +179 -1: /Documents/GitHub/apidash/test/providers/ui_providers_test.dart: Testing navRailIndexStateProvider UI should update correctly when navRailIndexStateProvider changes [E]
  Test failed. See exception logs above.
  The test description was: UI should update correctly when navRailIndexStateProvider changes

To run this test again: /Documents/flutter/bin/cache/dart-sdk/bin/dart test /Documents/GitHub/apidash/test/providers/ui_providers_test.dart -p vm --plain-name 'Testing navRailIndexStateProvider UI should update correctly when navRailIndexStateProvider changes'