tests: add a test for the sink issue

tests/lang_types/tuples/tsink_generic_tuple_assignment.nim

@@ -0,0 +1,24 @@
+discard """
+  description: '''
+    Regression test for a bug where assigning a ``sink`` generic tuple type
+    to a variable of non-``sink`` type led to the value being shallow copied.
+  '''
+  matrix: "--showir:mir_in:test --hints:off"
+  nimout: '''
+-- MIR: test
+scope:
+  def x: sink Tuple[system.int]
+  def v: Tuple[system.int]
+  v = sink x
+
+-- end
+'''
+"""
+
+type Tuple[T] = (T, T)
+
+proc test(x: sink Tuple[int]) =
+  var v: Tuple[int]
+  v = x # this assignment was considered to involve a conversion
+
+test default(Tuple[int])