cmd/cue: add test for filetype attributes

Currently, arbitrary key-value pairs can be specified in a filetype,
but this isn't ideal behavior.

Add a test for this, to be fixed in a subsequent CL.

For #3283.

Signed-off-by: Roger Peppe <[email protected]>
Change-Id: I2d18509e116f427a8eca19899329df0e68ab2255
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1200900
Reviewed-by: Daniel Martí <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>

Author: rogpeppe

cmd/cue/cmd/testdata/script/unknown_filetype_attrval.txtar

@@ -0,0 +1,10 @@
+# Check that we cannot specify an arbitary unknown key-value pair
+# in a filetype.
+# TODO fix this test.
+
+exec cue export cue+foo=bar: x.cue
+# cmp stderr expect-stderr
+-- x.cue --
+true
+-- expect-stderr --
+unknown filetype foo