There is no "trial" version of Windows 7.
You can obtain a legitimate Windows 7 ISO file by using the download tool here: https://www.heidoc.net/joomla/technology-science/microsoft/67-microsoft-windows-and-office-iso-download-tool
If you are willing to use an ISO file from August 2018 (5.5 GB), you can download that immediately.
However, if you want the original version, downloads are quite limited. Notes below from the Heidoc support forum.
- "The download database on the server will be reset on the first day of every month. We will then generate one link per hour on a first come first served basis, whatever is requested, until all licenses are used up. The database is then again reset on the first day of the following month, and downloads can be provided again for some time."
- "The Aug 2018 Win7 downloads are "standalone" edition downloads (only come with Home Premium, Pro & Ultimate as separate editions and not bundled together) that include IE11 & .NET 4.7.2 but are available in US English language only. all post sp1 updates except the big KB3125574 rollup update are included."
- "I suggest you use the original version (NOT Aug 2018) since it is the official one and it can be unlocked to become a multi-SKU disc image. The only downside is you have to install all updates after the installation." https://www.wincert.net/forum/topic/14731-is-it-just-me-or-is-the-official-client-fixed-resolutioned/
- https://www.wincert.net/forum/topic/14688-question-about-win7-iso/
For additional details, see: https://www.wincert.net/forum/topic/14659-win-7-office-2010-monthly-download-days-starting-october-1-experimental/
You can install Windows using these ISO files without a product key. You will have a 30-day "grace period" before activation is required (and that can be extend a bit if necessary; see, e.g., https://www.howtogeek.com/howto/windows-7/extend-the-windows-7-trial-from-30-to-120-days/). That should be long enough for you to do whatever testing you need to do.