Work around limitation for JDK12+ about j.l.constant.Constable (#1941)

`java.lang.constant.Constable` was introduced at JDK12 and is a
super trait of core boxed classes. This causes code like

    val seq = Seq(Double.box(1d), "")
    val elem = seq.head

to infer types that mention `Constable`: `seq` is of type
`Seq[Constable with ...]` and `elem` of type `Constable with ...`.
The latter erases to `Constable`, which results in linking errors.

It is unfortunately impossible to correctly implement `Constable`
in Scala Native, because it requires reflection, so the general
issue is not fixable.

This commit works around the issue in our own test suite, by
ascribing some values with explicit types, so that `Constable` does
not appear in erased types.

unit-tests/src/test/scala/java/util/DefaultFormatterTest.scala

@@ -830,7 +830,7 @@ class DefaultFormatterTest {
   }
 
   @Test def formatForGeneralConversionType_bB(): Unit = {
-    val triple = Array(
+    val triple: Array[Array[Object]] = Array(
       Array(Boolean.box(false), "%3.2b", " fa"),
       Array(Boolean.box(false), "%-4.6b", "false"),
       Array(Boolean.box(false), "%.2b", "fa"),
@@ -891,7 +891,7 @@ class DefaultFormatterTest {
   }
 
   @Test def formatForGeneralConversionType_sS(): Unit = {
-    val triple = Array(
+    val triple: Array[Array[Object]] = Array(
       Array(Boolean.box(false), "%2.3s", "fal"),
       Array(Boolean.box(false), "%-6.4s", "fals  "),
       Array(Boolean.box(false), "%.5s", "false"),

unit-tests/src/test/scala/java/util/FormatterUSTest.scala

@@ -829,7 +829,7 @@ class FormatterUSTest {
   }
 
   @Test def formatForGeneralConversionType_bB(): Unit = {
-    val triple = Array(
+    val triple: Array[Array[Object]] = Array(
       Array(Boolean.box(false), "%3.2b", " fa"),
       Array(Boolean.box(false), "%-4.6b", "false"),
       Array(Boolean.box(false), "%.2b", "fa"),
@@ -921,7 +921,7 @@ class FormatterUSTest {
   }
 
   @Test def formatForGeneralConversionType_sS(): Unit = {
-    val triple = Array(
+    val triple: Array[Array[Object]] = Array(
       Array(Boolean.box(false), "%2.3s", "fal"),
       Array(Boolean.box(false), "%-6.4s", "fals  "),
       Array(Boolean.box(false), "%.5s", "false"),