tests: runtime: processor_content_modifier: add test cases for convert

[nokute78]

This patch is to enrich test cases for convert of processor_content_modifier.

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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

• Backport to latest stable release.

Debug/Valgrind output

d$ valgrind --leak-check=full bin/flb-rt-processor_content_modifier 
==48356== Memcheck, a memory error detector
==48356== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==48356== Using Valgrind-3.18.1 and LibVEX; rerun with -h for copyright info
==48356== Command: bin/flb-rt-processor_content_modifier
==48356== 
Test logs.action.insert...                      [ OK ]
Test logs.action.delete...                      [ OK ]
Test logs.action.rename...                      [ OK ]
Test logs.action.upsert...                      [ OK ]
Test logs.action.hash...                        [ OK ]
Test logs.action.extract...                     [ OK ]
Test logs.action.convert_from_string_to_int...  [ OK ]
Test logs.action.convert_from_int_to_string...  [ OK ]
Test logs.action.convert_from_string_to_double... [ OK ]
Test logs.action.convert_from_double_to_string... [ OK ]
Test logs.action.convert_from_string_to_boolean... [ OK ]
Test logs.action.convert_from_int_to_boolean... [ OK ]
Test logs.action.convert_from_int_to_double...  [ OK ]
Test logs.action.convert_from_double_to_int...  [ OK ]
Test logs.action.convert_from_double_to_boolean... [ OK ]
Test logs.action.convert_from_null_to_string... [ OK ]
Test logs.action.convert_from_null_to_int...    [ OK ]
Test logs.action.convert_from_null_to_double... [ OK ]
SUCCESS: All unit tests have passed.
==48356== 
==48356== HEAP SUMMARY:
==48356==     in use at exit: 0 bytes in 0 blocks
==48356==   total heap usage: 32,830 allocs, 32,830 frees, 14,408,994 bytes allocated
==48356== 
==48356== All heap blocks were freed -- no leaks are possible
==48356== 
==48356== For lists of detected and suppressed errors, rerun with: -s
==48356== 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.