Test enhancement

[peter279k]

Changed log

• Since the php-5.x versions are inactive for about long time.
  Using the php-7.x versions is fine.
• Letting the build/ folder be under .gitignore file because this file is generated by PHPUnit.
• Using the PHPUnit\Framework\TestCase namespace to be compatible with future PHPUnit versions.
• The PHPUnit fixtures are about setUp and tearDown methods are protected.
• Using assertCount to assert expected length is same as result count.
• Using the assertInternalType to assert expected is array type.
• The --dev option is deprecated when executing composer install command.
  The deprecated warning message is as follows:

You are using the deprecated option "dev". Dev packages are installed by default now.