[Python] Skip PyArrow tests when a testing component is not installed

### Describe the enhancement requested

As an example, when `Compute` is included in the build but `UTF8Proc` is not, you get failures in the compute tests for the utf-8 modules:

> FAILED pyarrow/tests/test_compute.py::test_string_py_compat_boolean[utf8-is_lower] - AttributeError: module 'pyarrow.compute' has no attribute 'utf8_is_printable'. Did you mean: 'ascii_is_printable'?
FAILED pyarrow/tests/test_compute.py::test_string_py_compat_boolean[utf8-is_numeric] - AttributeError: module 'pyarrow.compute' has no attribute 'utf8_is_printable'. Did you mean: 'ascii_is_printable'?

These functions might be helpful:
https://github.com/apache/arrow/blob/925cbd81427ae02ce897c406a264d53c8813b920/python/pyarrow/__init__.py#L96
https://github.com/apache/arrow/blob/925cbd81427ae02ce897c406a264d53c8813b920/python/pyarrow/__init__.py#L119

### Component(s)

Python

Start with the failing cases in pyarrow/tests/test_compute.py and inspect the referenced component-related functions in python/pyarrow/__init__.py. Verify how unavailable testing components are represented, then ensure the affected tests are skipped when the component is absent and run the relevant PyArrow compute tests.

Written by the indexing model from the issue text.