Is there a tester branch for Manjaro ARM?
Just like x86_64 - the arm architecture has three branches.
For a lot of reasons the recommended method to get an up-to-date image is to clone the manjaro-arm-installer script.
Is there an installation disk image for ARM?
There is no installation disk image.
Any image is per device images e.g. for rpi - some is actively maintained - others not.
The images available is not installer images - although some of the desktop images provide an oem installer to setup initial username and password.
If you have to find the Download ARM – Manjaro then be warned - most - if not all - is from february 2023 - while they are likely to work - it may - will - require some effort to update - thus you should really consider using the installer script mentioned earlier to create an up-to-date image.
I already noticed that. I last updated a stable release of Manjaro (X86_64 platform) on one of my laptops years ago and it was not easy to update.