fix: properly handle XTestGoFiles

[fredrikaverpil]

Why this change?

If you add tests as part of [packagename]_test (an otherwise non-existing
package), go list -json did not properly aid in mapping from filename to Go
package, and cause the go test command to fall back to using ./... instead
of the desired package name. This become a problem if you want to run all tests
of a given file.

What was changed?

Not only look for the TestGoFiles key in the JSON output from go list -json.
Also look for the XTestGoFiles key.

Notes

To do

• Fix.
• Tests.

TestGoFiles vs XTestGoFiles

TestGoFiles:

• Contains test files that are part of the same package as the code being
  tested.
• These are files named *_test.go that have the same package name as the
  source files.
• Used for white-box testing where tests can access unexported (private)
  members.

XTestGoFiles:

• Contains test files that are in a separate package named [packagename]_test.
• These are files named *_test.go that have the package name
  [packagename]_test.
• Used for black-box testing where tests can only access exported (public)
  members.
• This field is absent in the JSON output when there are no external test files.

Example:

// main.go
package mypackage

func Add(a, b int) int { return a + b }
func internal() string { return "private" }

// internal_test.go (TestGoFiles)
package mypackage

func TestAdd(t *testing.T) {
    // Can access both Add() and internal()
}

// external_test.go (XTestGoFiles)
package mypackage_test

func TestAdd(t *testing.T) {
    // Can only access Add(), cannot access internal()
}

---

fixes: #222