As a junior developer, I'm working in a company that develops software for the airline industry. We have a test team, so I don't have any motivation to learn testing software. My friend is working for a small company as an back-end developer. Their team doesn't have any specific test team, and they do their tests on their own. Should a back-end developer learn about testing software?
Absolutely and unequivocally: yes!
It's a core skill which you will be expected to have at a large percentage of companies you'll want to work for in the future.
As a developer, the technical aspects of testing are more interesting than than the methodological ones: learn using a unit testing framework, set up automated testing, try doing test-driven development to see how you like it.
If you want to specialize in it, performance/stress testing and security/penetration testing are quite sought-after skills.
Testing the code you are working on is part of the discipline that makes a good programmer into a great programmer. Setting up automatic testing, using light-weight, fast test cases allows you to establish a base-line of functionality; modifying the code to fix errors or add new functionality can be incrementally tested and gives you rapid feedback on your code. This rapid feedback, when done frequently, is much more efficient in terms of re-work than building it all, compiling and testing, because you end up not going so far off the right path.