cmd/cue: add test case for issue 3262

This CL adds the test case for editing a module
file with a module path that doesn't have a major
version suffix. The behavior will be fixed in the next
CL in the chain.

For #3262

Signed-off-by: Roger Peppe <rogpeppe@gmail.com>
Change-Id: I1b26c462cf7e23240959eb904197dd12ee109479
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1197628
Reviewed-by: Paul Jolly <paul@myitcv.io>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>

cmd/cue/cmd/testdata/script/modedit_nomajorversion.txtar

@@ -0,0 +1,18 @@
+# Test that we can use cue mod edit to change
+# set a module path that does not contain a major
+# version suffix.
+
+exec cue mod init test.example
+cmp cue.mod/module.cue want-module
+
+# TODO this should not fail.
+! exec cue mod edit --module other.example
+cmp stderr want-stderr
+
+-- want-module --
+module: "test.example"
+language: {
+	version: "v0.10.0"
+}
+-- want-stderr --
+invalid argument "other.example" for "--module" flag: malformed module path "other.example": no major version found in module path