Improve test output and code sharing

[tommaisey]

While working on a feature I noticed that test output could be more helpful. It now prints the expected and actual results for a failed test.

I also deduplicated some code that was copied across several tests, and made some uses of global TEST variables into locals.

Example output:

test/crossfile/hover.lua:32: 
=================
test failed. got:
=================
```lua
function f(x: number)
```
=================
expected:
=================
```lua
function f(x: number)
```

---

```lua
function f(x: number, y: number)
```
=================

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the testing framework by centralizing comparison logic and improving error reporting for failed assertions. It also cleans up test code by removing redundant comparison functions and localizing test helper functions, leading to more maintainable and informative tests.

Highlights

• Improved Test Output: Test failures now provide more helpful information by printing both the expected and actual results, making it easier to diagnose issues.
• Centralized Test Comparison Logic: A new compare module was introduced to centralize the eq (equality) function and EXISTS constant, removing duplicated logic across various test files.
• Code Deduplication and Refactoring: Duplicated eq functions were removed from multiple test files, and several global TEST functions were converted to local functions, improving code organization and reducing global scope pollution.

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[gemini-code-assist]

Code Review

This pull request refactors test utility functions by extracting a common compare module. This module provides a deep equality comparison function (compare.eq) and an EXISTS constant. Multiple test files (test/code_action/init.lua, test/completion/init.lua, test/crossfile/completion.lua, test/crossfile/hover.lua, test/crossfile/infer.lua, test/crossfile/references.lua) are updated to import and use this new compare module, removing their duplicate local implementations of eq and EXISTS. A minor improvement opportunity was identified in test/compare.lua regarding a misleading error message in the eq function.

[gemini-code-assist]

The error message here seems a bit misleading. This loop checks for keys present in the result (b) but not in the expected (a). Therefore, it's an "unexpected" key, not a "missing" one. I suggest changing the message to reflect that.

                return false, string.format(".%s: unexpected key in result", k)

[sumneko]

Thank you!