add error test

Author: mununki

jscomp/build_tests/super_errors/expected/variant_coercion_bigint.res.expected

@@ -0,0 +1,10 @@
+
+  We've found a bug for you!
+  /.../fixtures/variant_coercion_bigint.res:5:10-20
+
+  3 │ let x = One(true)
+  4 │ 
+  5 │ let y = (x :> bigint)
+  6 │ 
+
+  Type x is not a subtype of bigint

jscomp/build_tests/super_errors/expected/variant_coercion_bigint_as.res.expected

@@ -0,0 +1,10 @@
+
+  We've found a bug for you!
+  /.../fixtures/variant_coercion_bigint_as.res:5:10-20
+
+  3 │ let x = One
+  4 │ 
+  5 │ let y = (x :> bigint)
+  6 │ 
+
+  Type x is not a subtype of bigint

jscomp/build_tests/super_errors/fixtures/variant_coercion_bigint.res

@@ -0,0 +1,5 @@
+type x = | @as(1n) One(bool) | @as(2n) Two
+
+let x = One(true)
+
+let y = (x :> bigint)

jscomp/build_tests/super_errors/fixtures/variant_coercion_bigint_as.res

@@ -0,0 +1,5 @@
+type x = | @as(1n) One | Two
+
+let x = One
+
+let y = (x :> bigint)