Add sumtype to Phobos (#7702)

Add sumtype to Phobos
merged-on-behalf-of: unknown

test/betterc_module_tests.d

@@ -0,0 +1,30 @@
+static immutable bettercModules = [
+    "std.sumtype"
+];
+
+template from(string modname)
+{
+    mixin("import from = ", modname, ";");
+}
+
+void testModule(string modname)()
+{
+    import core.stdc.stdio : printf;
+
+    printf("Running BetterC tests for %.*s\n", cast(int) modname.length, modname.ptr);
+
+    static foreach (test; __traits(getUnitTests, from!modname))
+    {
+        test();
+    }
+}
+
+extern(C) int main()
+{
+    static foreach (modname; bettercModules)
+    {
+        testModule!modname;
+    }
+
+    return 0;
+}