// empty expression matches any string.
// For example, using "matching" to mean "whose name contains":
//
//	go test -run ''        # Run all tests.
//	go test -run Foo       # Run top-level tests matching "Foo", such as "TestFooBar".
//	go test -run Foo/A=    # For top-level tests matching "Foo", run subtests matching "A=".
//	go test -run /A=1      # For all top-level tests, run subtests matching "A=1".