The goal is to find bugs while leaving time and money to do other things too. So a high-quality test is a test that finds bugs but doesn’t require a disproportionate expense to write, maintain, or operate.
Sometimes you can tell whether a test is low-quality just by looking at it. For example, if the test is buggy or written in a brittle way so that every change to the system requires a change to the test, it’s a low-quality test.
You can’t tell whether a test is high-quality without measuring the test’s impact, i.e. measuring whether it finds bugs. A test can be really well-written but if it doesn’t find bugs, it’s not a good use of resources.
Of course, it’s hard to know in advance whether a test will be high-quality because it’s hard to know where the bugs will be.