Fix LD_LIBRARY_PATH handling for shared libraries during fpm test

[ZainabTravadi]

When running fpm test for projects using shared libraries,
the compiled .so file may not be found because the directory
containing the library is not included in LD_LIBRARY_PATH.

This change ensures that shared library output directories
(target%output_dir) are included when collecting library
paths via get_library_dirs().

As a result, test executables can locate shared libraries
without requiring users to manually configure LD_LIBRARY_PATH.

Validation:

• All existing fpm tests pass
• Verified using a minimal fpm project with type="shared"
• The test executable successfully located the .so
  without manually setting LD_LIBRARY_PATH

[Copilot]

Pull request overview

Ensures fpm test can locate shared libraries by including target output directories in the paths gathered by get_library_dirs() (so LD_LIBRARY_PATH includes the .so directory during test execution).

Changes:

• Initializes shared_lib_dirs as empty and adds clarifying comments.
• Filters considered targets in get_library_dirs() and adds target%output_dir while deduplicating.

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[Copilot]

The PR description focuses on fixing runtime discovery for shared libraries, but this change also includes FPM_TARGET_ARCHIVE. Static archives don’t affect LD_LIBRARY_PATH, so either (a) restrict the condition to FPM_TARGET_SHARED only, or (b) update the PR description to explicitly state why archive targets should be included here.

[Copilot]

These comments are misleading with the current logic: the code path also applies to FPM_TARGET_ARCHIVE due to the filter above. Please adjust the wording to match the implementation (or adjust the implementation to match the comment).

[Copilot]

Using all(target%target_type /= [ ... ]) on a scalar-to-array comparison is correct but non-obvious to many readers. Consider rewriting this as a clearer membership test (e.g., negate an any(target%target_type == [...])) to reduce the chance of misinterpretation during future maintenance.

Suggested change

	if (all(target%target_type /= [FPM_TARGET_SHARED,FPM_TARGET_ARCHIVE])) cycle
	if (.not. any(target%target_type == [FPM_TARGET_SHARED,FPM_TARGET_ARCHIVE])) cycle

[ZainabTravadi]

I updated the implementation to only consider FPM_TARGET_SHARED targets and replaced the condition with a clearer any() membership check as suggested. The comments were also updated to reflect that only shared libraries affect LD_LIBRARY_PATH.