test: add brotli_compress_add test case

tests/compress_add.phpt

@@ -0,0 +1,42 @@
+--TEST--
+Test brotli_compress_add() functionality
+--SKIPIF--
+<?php
+if (!extension_loaded('brotli')) die('skip need ext/brotli');
+if (PHP_VERSION_ID < 70000) die('skip need version: 7.0+');
+?>
+--FILE--
+<?php
+
+$brotliContext = brotli_compress_init();
+
+$strings = [
+  'Hello, how are you? How is it going?' . PHP_EOL,
+  'I am fine thanks' . PHP_EOL,
+  'Hello, how are you? How is it going?' . PHP_EOL,
+];
+
+$compressed = '';
+foreach ($strings as $string) {
+  $compressedString = brotli_compress_add($brotliContext, $string);
+  if (strlen($compressedString) === 0) {
+    echo "Incremental compress failed\n";
+  } elseif ($compressedString === $string) {
+    // temporary only for test
+    echo "Incremental compress failed to compress\n";
+  } else {
+    $compressed .= $compressedString;
+  }
+}
+
+$compressed .= brotli_compress_add($brotliContext, '', BROTLI_FINISH);
+if (brotli_uncompress($compressed) !== implode('', $strings)) {
+  echo "Compression invalid\n";
+} else {
+  echo "OK\n";
+}
+?>
+===DONE===
+--EXPECTF--
+OK
+===DONE===