[TASK] Add some basic tests for CSSString (#999)

Part of #757

Author: oliverklee

config/phpstan-baseline.neon

@@ -414,6 +414,12 @@ parameters:
 			count: 1
 			path: ../src/RuleSet/RuleSet.php
 
+		-
+			message: '#^Parameters should have "string" types as the only types passed to this method$#'
+			identifier: typePerfect.narrowPublicClassMethodParamType
+			count: 1
+			path: ../src/Value/CSSString.php
+
 		-
 			message: '#^Loose comparison via "\!\=" is not allowed\.$#'
 			identifier: notEqual.notAllowed

tests/Unit/Value/CSSStringTest.php

@@ -0,0 +1,84 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Sabberworm\CSS\Tests\Unit\Value;
+
+use PHPUnit\Framework\TestCase;
+use Sabberworm\CSS\Value\CSSString;
+use Sabberworm\CSS\Value\PrimitiveValue;
+use Sabberworm\CSS\Value\Value;
+
+/**
+ * @covers \Sabberworm\CSS\Value\CSSString
+ * @covers \Sabberworm\CSS\Value\PrimitiveValue
+ * @covers \Sabberworm\CSS\Value\Value
+ */
+final class CSSStringTest extends TestCase
+{
+    /**
+     * @test
+     */
+    public function isPrimitiveValue(): void
+    {
+        $subject = new CSSString('');
+
+        self::assertInstanceOf(PrimitiveValue::class, $subject);
+    }
+
+    /**
+     * @test
+     */
+    public function isValue(): void
+    {
+        $subject = new CSSString('');
+
+        self::assertInstanceOf(Value::class, $subject);
+    }
+
+    /**
+     * @test
+     */
+    public function getStringReturnsStringProvidedToConstructor(): void
+    {
+        $string = 'coffee';
+        $subject = new CSSString($string);
+
+        self::assertSame($string, $subject->getString());
+    }
+
+    /**
+     * @test
+     */
+    public function setStringSetsString(): void
+    {
+        $subject = new CSSString('');
+        $string = 'coffee';
+
+        $subject->setString($string);
+
+        self::assertSame($string, $subject->getString());
+    }
+
+    /**
+     * @test
+     */
+    public function getLineNoByDefaultReturnsZero(): void
+    {
+        $subject = new CSSString('');
+
+        self::assertSame(0, $subject->getLineNo());
+    }
+
+    /**
+     * @test
+     */
+    public function getLineNoReturnsLineNumberProvidedToConstructor(): void
+    {
+        $lineNumber = 42;
+
+        $subject = new CSSString('', $lineNumber);
+
+        self::assertSame($lineNumber, $subject->getLineNo());
+    }
+}