Adjust the existing Lua style guide

Considering the existing Lua style guide there is no mention for our test naming policy. This led to a little arguing in our patch list.

@Gerold103 claimed that we agreed to the following some time ago:

gh-####-description.test.lua

However, since we backport bugs related to the issues from other queues, the gh-#### prefix is ambigious. Thereby I guess the following pattern is the most convenient one for LuaJIT related tests:

<origin-issue-repo>-####-description.test.lua

Here are some examples:

• LuaJIT-505-fold-bug-in-string-find.test.lua
• tarantool-3196-bug-with-zero-hash-strings.test.lua

@kyukhin, @Totktonada, @Gerold103, @kostja, please dump your thoughts related to the naming policy here for the further Lua style guide amending.

Furthermore, all existing tests in LuaJIT suite seem to be written considering the current guideline and violate several practices mentioned by @Totktonada in the review:

• Use local for variables (tap, test).
• Use os.exit(test:check() and 0 or 1) at the end.

Please consider applying these comments to the example in out style guide.

Read the existing Lua style guide's testing section and the linked patch-list discussion first, since the naming policy is still presented as a proposal. Then inspect the LuaJIT suite under test to compare its conventions with the cited review comments. Done means the policy is agreed and the guide's test-naming examples and Lua test practices reflect that decision.