Add some tests

tests/neg-custom-args/captures/capset-bound.scala

@@ -0,0 +1,18 @@
+import caps.*
+
+class IO
+
+case class File(io: IO^)
+
+def test(io1: IO^, io2: IO^) =
+  def f[C >: CapSet^{io1} <: CapSet^](file: File^{C^}) = ???
+  val f1: File^{io1} = ???
+  val f2: File^{io2} = ???
+  val f3: File^{io1, io2} = ???
+  f[CapSet^{io1}](f1)
+  f[CapSet^{io1}](f2) // error
+  f[CapSet^{io1}](f3) // error
+  f[CapSet^{io2}](f2) // error
+  f[CapSet^{io1, io2}](f1)
+  f[CapSet^{io1, io2}](f2)
+  f[CapSet^{io1, io2}](f3)

tests/neg-custom-args/captures/i21868.scala

@@ -0,0 +1,14 @@
+import caps.
+
+trait AbstractWrong:
+    type C <: CapSet
+    def boom(): Unit^{C^} // error
+
+trait Abstract:
+    type C <: CapSet^
+    def boom(): Unit^{C^}
+
+class Concrete extends Abstract:
+    type C = Nothing
+    def boom() = () // error
+