Bugfix `-fserial-variable`

[tsh-hashimoto]

Related to #497

This pull request introduces several changes across multiple files to enhance the handling of serial variables in the codebase. The most important changes include updates to the joutput_field_storage function, the addition of new test cases, and modifications to existing test files to include the new serial variable checks.

Codebase improvements:

• cobj/codegen.c: Updated joutput_field_storage to handle cb_flag_serial_variable by generating Java identifiers for serial variables.

Test enhancements:

• tests/Makefile.am: Added misc.src/fserial-variable.at to the list of dependencies to ensure the new test case is included. [1] [2]
• tests/command-line-options.src/fserial-variable.at: Added new test cases to verify the correct generation of serial variable identifiers in the output. [1] [2]
• tests/misc.at: Included fserial-variable.at in the miscellaneous tests to ensure comprehensive coverage.

[tsh-hashimoto]

All tests have been passed. Please take a look.