tests: internal: log_event_decoder: add test for log_event_decoder

[nokute78]

This patch is to add test code for flb_log_event_decoder.

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Enter [N/A] in the box, if an item is not applicable to your change.

Testing
Before we can approve your change; please submit the following in a comment:

• [N/A] Example configuration file for the change
• Debug log output from testing the change

• Attached Valgrind output that shows no leaks or memory corruption was found

If this is a change to packaging of containers or native binaries then please confirm it works for all targets.

• [N/A] Run local packaging test showing all targets (including any new ones) build.
• [N/A] Set ok-package-test label to test for all targets (requires maintainer to do).

Documentation

• [N/A] Documentation required for this feature

Backporting

• [N/A] Backport to latest stable release.

Debug/Valgrind output

$ valgrind --leak-check=full bin/flb-it-log_event_decoder 
==50620== Memcheck, a memory error detector
==50620== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==50620== Using Valgrind-3.18.1 and LibVEX; rerun with -h for copyright info
==50620== Command: bin/flb-it-log_event_decoder
==50620== 
Test create_destroy...                          [ OK ]
==50620== Warning: invalid file descriptor -1 in syscall close()
Test init_destroy...                            [ OK ]
==50620== Warning: invalid file descriptor -1 in syscall close()
Test decode_timestamp...                        [ OK ]
==50620== Warning: invalid file descriptor -1 in syscall close()
Test decode_object...                           [ OK ]
==50620== Warning: invalid file descriptor -1 in syscall close()
Test decoder_next...                            [ OK ]
==50620== Warning: invalid file descriptor -1 in syscall close()
SUCCESS: All unit tests have passed.
==50620== 
==50620== HEAP SUMMARY:
==50620==     in use at exit: 0 bytes in 0 blocks
==50620==   total heap usage: 4,229 allocs, 4,229 frees, 538,370 bytes allocated
==50620== 
==50620== All heap blocks were freed -- no leaks are possible
==50620== 
==50620== For lists of detected and suppressed errors, rerun with: -s
==50620== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

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Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.