Added two regression test scripts.

[vishnu350]

@ivan-hc, @fiftydinar.

These are two simple tests that can be run independently by testers on their systems to help with regression testing. They will display either pass or fail to help with debugging.

The idea is we should add more tests here to catch problems that occur in the future like sandboxing failure etc, and run all of them before triggering a release.

Added two sample regression scripts.

1. am_install_test.sh - Installs two apps with the same name and checks if they are listed correctly.
2. am_checksum_test.sh - Installs an app with known checksum verified status and checks if it was verified correctly.

NOTE: I hardcoded zsync2 in the test script because it is a small download. Usually test devs will randomize this for testing, but what if it randomly picks blender (1GB file).

[ivan-hc]

I noticed that in some steps you use am --user, but this wont work if you don't set an AppMan configuration file first, in ~/.config/appman/appman-config

Anyway, this is a good idea. I will keep this opened for new implementations.

[fiftydinar]

Good start, but for a reference, AM-CHECK is an example of regression tests done right.

They are complex, but they are really good.

The idea would be to run the regression scripts in GitHub action and get a report if there is a regression or not.
It can be set to run on every PR made, where you can see the checkmark if PR makes a regression or not.