Fix locale discovery so provider tests run again

[a-vasyukov]

What is the reason for this PR?

• A new feature
• Fixed an issue

Faker\Test\TestCase::getAllLocales() globs __DIR__ . '/../../src/Faker/Provider/*_*'. On 2.0
that path resolves outside the repository, so the glob returns an empty array,
localeDataProvider() yields no data sets, and every test using it is silently skipped — PHPUnit
only reports skipped by data provider.

Both halves of the path went stale in the same commit — 67825a1 "Fix: Flatten directory structure"
(#773, 2023-09-20): tests moved from test/Faker/ to test/, and providers moved from
src/Faker/Provider to src/Provider. The glob was never updated, so these tests have not run
since then.

This affects 8 test methods in ProviderOverrideTest, 4 in InternetTest and 1 in PaymentTest,
across all 75 locales. On 1.24 the same code lives in test/Faker/TestCase.php, where the path
still resolves, so that branch is unaffected.

To reproduce on 2.0 before this change:

vendor/bin/phpunit --filter ProviderOverrideTest
# 8 tests, every one of them "skipped by data provider"

Author's checklist

• Follow the Contribution Guide
• New features and changes are documented

Summary of changes

• point the glob at src/Provider relative to test/
• add a regression test asserting that locales are discovered; it fails with
  No locales were discovered if the path breaks again

Before:

Tests: 1416, Assertions: 10495, Skipped: 16.

After:

Tests: 2453, Assertions: 12457, Skipped: 34.

No test starts failing once they run — the suite stays green, only the count grows.

Review checklist

• All checks have passed
• Changes are added to the CHANGELOG.md
• Changes are approved by maintainer

[a-vasyukov]

Two weeks, no CI run — workflows on PRs from outside contributors need manual approval, so the
1416 → 2453 jump is still verified locally only.

Anything you'd like changed here? If patching the path by hand looks fragile, I can resolve the
package root instead, so a future directory move can't disable the data provider silently again.