fix Config::get_self_url() invoked from plugin context, better deal with multiple trailing slashes in URL, update phpunit image path

tests/ConfigTest.php

@@ -104,6 +104,18 @@ final class SelfUrlPathTest extends TestCase {
 		);
 	}
 
+	public function test_self_url_i(): void {
+		$_SERVER = [];
+
+		$_SERVER["HTTP_HOST"] = "example.com";
+		$_SERVER["REQUEST_URI"] = "/tt-rss////plugins.local/example///api/long path/test.php";
+
+		$this->assertEquals(
+			'http://example.com/tt-rss',
+			Config::get_self_url(true)
+		);
+	}
+
 	public function test_get_self_dir(): void {
 		$this->assertEquals(
 			dirname(__DIR__), # we're in (app)/tests/