QA: use the most specific PHPUnit assertion possible

This is a long established best practice.

PHPUnit contains a variety of assertions and the ones available have been extended  hugely over the years.
To have the most reliable tests, the most specific assertion should be used.

This implements this for the Yoast ACF Analysis plugin.

Refs:
* https://phpunit.de/manual/4.8/en/appendixes.assertions.html
* https://phpunit.readthedocs.io/en/7.5/assertions.html#

Most notably, this changes calls to `assertEquals()` to `assertSame()`, where `assertEquals()` does a loose type comparison and `assertSame()` does a strict type comparison.

Refs:
* https://phpunit.de/manual/6.5/en/appendixes.assertions.html#appendixes.assertions.assertEquals
* https://phpunit.de/manual/6.5/en/appendixes.assertions.html#appendixes.assertions.assertSame

It also includes a stricter check on arrays which are supposed to be empty and removes a duplicate function call.

tests/php/unit/Configuration/configuration-test.php

@@ -59,7 +59,7 @@ public function testEmpty() {
 			$configuration->to_array()
 		);
 
-		$this->assertEquals( Filters\applied( 'acf/get_info' ), 1 );
+		$this->assertSame( Filters\applied( 'acf/get_info' ), 1 );
 	}
 
 	/**
@@ -78,7 +78,7 @@ public function testACF5VersionFunction() {
 		);
 		$config        = $configuration->to_array();
 
-		$this->assertEquals( $acf_version, $config['acfVersion'] );
+		$this->assertSame( $acf_version, $config['acfVersion'] );
 	}
 
 	/**

tests/php/unit/Configuration/string-store-test.php

@@ -22,8 +22,11 @@ protected function getStore() {
 	 * @return void
 	 */
 	public function testEmpty() {
-		$store = $this->getStore();
-		$this->assertEmpty( $store->to_array() );
+		$store  = $this->getStore();
+		$result = $store->to_array();
+
+		$this->assertInternalType( 'array', $result );
+		$this->assertEmpty( $result );
 	}
 
 	/**
@@ -101,7 +104,11 @@ public function testAddNonString() {
 		$store = $this->getStore();
 
 		$this->assertFalse( $store->add( 999 ) );
-		$this->assertEmpty( $store->to_array() );
+
+		$result = $store->to_array();
+
+		$this->assertInternalType( 'array', $result );
+		$this->assertEmpty( $result );
 	}
 
 	/**
@@ -127,7 +134,10 @@ public function testRemove() {
 
 		$store->remove( $type_b );
 
-		$this->assertEmpty( $store->to_array() );
+		$result = $store->to_array();
+
+		$this->assertInternalType( 'array', $result );
+		$this->assertEmpty( $result );
 	}
 
 	/**

tests/php/unit/main-test.php

@@ -42,7 +42,9 @@ public function testInvalidConfig() {
 		$testee = new \AC_Yoast_SEO_ACF_Content_Analysis();
 		$testee->boot();
 
-		$this->assertNotSame( 'Invalid Config', $registry->get( 'config' ) );
-		$this->assertInstanceOf( \Yoast_ACF_Analysis_Configuration::class, $registry->get( 'config' ) );
+		$result = $registry->get( 'config' );
+
+		$this->assertNotSame( 'Invalid Config', $result );
+		$this->assertInstanceOf( \Yoast_ACF_Analysis_Configuration::class, $result );
 	}
 }