Bump 515 alternate test to 1610 (#3925)

# About the pull request

Now that 515.1609 is *finally* not a closed beta, this test can now be
bumped finally to a version we are using on the live server. Now
515.1610 since 1609 was busted.

# Explain why it's good for the game

Testing should best replicate what is being used on the live server.

# Testing Photographs and Procedure

See checks.

</details>

# Changelog
:cl: Drathek
code: Bump alternate 515 testing to byond 1610
/:cl:

.github/alternate_byond_versions.txt

@@ -6,4 +6,4 @@
 # Example:
 # 500.1337: runtimestation
 
-515.1603: lv624
+515.1610: lv624