Tests: Use assertIsCallable() for callable assertions.

[rootzero00]

Replaces eight generic assertTrue( is_callable() ) assertions with
PHPUnit's dedicated assertIsCallable() assertion in:

• tests/phpunit/tests/customize/widgets.php
• tests/phpunit/tests/multisite/wpCacheSwitchToBlogFallback.php

This is a test-only cleanup and does not change WordPress runtime behavior.

The multisite assertions were added as a review-driven follow-up after the
same pattern was identified in Tests_Multisite_WpCacheSwitchToBlogFallback.

Trac ticket: https://core.trac.wordpress.org/ticket/65819

Testing

• npm run test:php -- --filter Tests_WP_Customize_Widgets --colors=never --do-not-cache-result

  • PASS: 28 tests, 410 assertions

• npm run test:php -- -c tests/phpunit/multisite.xml --filter Tests_Multisite_WpCacheSwitchToBlogFallback::test_wp_cache_switch_to_blog_function_exists --colors=never --do-not-cache-result

  • PASS: 1 test, 4 assertions

• npm run test:php -- -c tests/phpunit/multisite.xml --filter Tests_Multisite_WpCacheSwitchToBlogFallback --colors=never --do-not-cache-result

  • PASS: 25 tests, 136 assertions

• npm run env:composer -- lint tests/phpunit/tests/multisite/wpCacheSwitchToBlogFallback.php

  • PASS

Fresh upstream verification confirmed that neither changed test file has
changed upstream since the candidate parent.

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Props raulsalvat, soean.

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[github-actions]

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[Soean]

We have the same pattern in Tests_Multisite_WpCacheSwitchToBlogFallback:

wordpress-develop/tests/phpunit/tests/multisite/wpCacheSwitchToBlogFallback.php

Lines 124 to 125 in ef4d2ff

	$this->assertTrue( is_callable( 'wp_cache_switch_to_blog' ) );
	$this->assertTrue( is_callable( 'wp_cache_switch_to_blog_fallback' ) );

[rootzero00]

Thanks — I've added those two occurrences in
Tests_Multisite_WpCacheSwitchToBlogFallback as a focused follow-up.

The additional change is limited to that one file (+2/-2), replacing the
two assertTrue( is_callable() ) assertions with assertIsCallable().

Fresh validation passes:

• targeted multisite method: 1 test, 4 assertions
• full multisite class: 25 tests, 136 assertions
• existing Customize class: 28 tests, 410 assertions
• PHP coding standards: PASS