Debugging tests with delve

I'm not familiar with delve, but if it can work on a compiled binary, just compile your tests using the -c flag:

    -c
        Compile the test binary to pkg.test but do not run it
        (where pkg is the last element of the package's import path).
        The file name can be changed with the -o flag.

Then run delve on the output.

Use dlv test:

$ dlv test -- -test.v
Type 'help' for list of commands.
(dlv) continue
=== RUN   TestReadFileError
--- PASS: TestReadFileError (0.00s)
=== RUN   TestReadFile
--- PASS: TestReadFile (0.00s)
[..]
PASS
Process 8014 has exited with status 0
(dlv) quit
Process 8014 has exited with status 0

You can also pass -test.run to select tests to run (just like go test -run).

Internally, this is the same as Flimzy's answer (it compiles the test binary with go test -c), but more streamlined and won't leave .test files for you to clean up.