Add test coverage

Alright, this is a decent start. The test cases that were omitted were...

  /**
   * Test case for overflow protection (almost overflowing).
   */
  public function testGetValueAlmostOverflowing() {
    $cloudflare_tags_header = $this->purgeTagsHeaders->current();
    static::assertInstanceOf(CloudflareTagsHeader::class, $cloudflare_tags_header);

    $this->config('cloudflare_purger.settings')
      ->set('max_response_header_length', 8192)
      ->save();

    // Run it right up to the brink. The breakdown of 1168 tags is 7008 hash
    // characters, plus 1167 comma characters, plus 12 characters for the
    // header name for a total of 8187 bytes.
    $tags = array_fill(0, 1168, 'test');

    $this->mockLoggerChannel
      ->expects(static::never())
      ->method('warning');

    // 1168 tags should yield 8175 bytes.
    static::assertSame(
      8175,
      strlen($cloudflare_tags_header->getValue($tags))
    );
  }

  /**
   * Test case for overflow protection (with slight overflow).
   */
  public function testGetValueSlightlyOverflowing() {

    $cloudflare_tags_header = $this->purgeTagsHeaders->current();
    static::assertInstanceOf(CloudflareTagsHeader::class, $cloudflare_tags_header);

    $this->config('cloudflare_purger.settings')
      ->set('max_response_header_length', 8192)
      ->save();

    // Run it right up to the brink. The breakdown of 1169 tags is 7014 hash
    // characters, plus 1168 comma characters, plus 12 characters for the
    // header name for a total of 8194 bytes.
    $tags = array_fill(0, 1169, 'test');

    $this->mockLoggerChannel
      ->expects(static::once())
      ->method('warning')
      ->with(
        'A response header was projected to exceed the configured limit by %amount tags. This may prevent content from purging properly.',
        ['%amount' => 1]
      );

    // The header value should be truncated to stay within the limit.
    static::assertSame(
      8175,
      strlen($cloudflare_tags_header->getValue($tags))
    );
  }

  /**
   * Test case for overflow protection (with gross overflow).
   */
  public function testGetValueGrosslyOverflowing() {

    $cloudflare_tags_header = $this->purgeTagsHeaders->current();
    static::assertInstanceOf(CloudflareTagsHeader::class, $cloudflare_tags_header);

    $this->config('cloudflare_purger.settings')
      ->set('max_response_header_length', 8192)
      ->save();

    // Run it right up to the brink. The breakdown of 2000 tags is 12000 hash
    // characters, plus 1999 comma characters, plus 12 characters for the
    // header name for a total of 14011 bytes.
    $tags = array_fill(0, 2000, 'test');

    $this->mockLoggerChannel
      ->expects(static::once())
      ->method('warning')
      ->with(
        'A response header was projected to exceed the configured limit by %amount tags. This may prevent content from purging properly.',
        ['%amount' => 832]
      );

    // The header value should be truncated to stay within the limit.
    static::assertSame(
      8175,
      strlen($cloudflare_tags_header->getValue($tags))
    );
  }

which really only make sense when there's a response header protection in place (another MR to come).