Unit tests for runtime effect SkSL errors found later in compilation

Forgot to add these when I fixed the bug. Verified that these tests
failed without the fix.

Bug: skia:10593
Change-Id: Ia20fad3cd8e5b0f63ca19946b8314eed49bec2bf
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/309716
Auto-Submit: Brian Osman <brianosman@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>

Author: brianosman

tests/SkRuntimeEffectTest.cpp

@@ -73,6 +73,10 @@ DEF_TEST(SkRuntimeEffectInvalid, r) {
     test("in shader child1; in shader child2;",
          "color = sample(p.x > 10 ? child1 : child2);",
          "expression");
+
+    // Errors that aren't caught until later in the compilation process (during optimize())
+    test("", "return; color.r = color.g;", "unreachable");
+    test("half badFunc() { }", "", "without returning");
 }
 
 DEF_TEST(SkRuntimeEffectInvalidColorFilters, r) {