feat(test): add parsing support for args field in [[test]] manifest

[ZainabTravadi]

Summary

This PR adds support for parsing an optional args field in [[test]] entries in fpm.toml.

Example:

[[test]]
name = "example"
args = ["--input", "file.txt"]

The parsed values are stored in test_config_t as an allocatable string array.

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Changes

• Extend test_config_t with args(:)

• Add get_args helper for parsing and validation

• Update new_test to parse the args field

• Update allowlist to accept "args"

• Add regression tests for:

  • valid args parsing
  • missing args field
  • invalid (non-array) args input

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Behavior

• args is optional
• If missing → remains unallocated
• If present → must be an array of strings
• Invalid types raise a syntax error

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Scope

This PR only adds manifest parsing support.

It does not modify:

• test execution
• command construction
• runtime behavior

Execution-level integration can be handled in a follow-up PR.

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Motivation

Currently, [[test]] entries do not support passing command-line arguments, limiting flexibility for parameterized tests.

This change introduces a small, backward-compatible improvement aligned with modern build system capabilities.

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Testing

Added regression tests in test_manifest.f90:

• test-args → verifies correct parsing
• test-args-missing → ensures optional behavior
• test-args-typeerror → validates error handling

All existing tests pass.

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Notes

• Keeps changes minimal and localized to manifest parsing
• Follows existing parsing and validation patterns
• Designed as an incremental step toward more flexible test configuration

Closes #1275