[cxx-interop] Use the right assertion function in tests

`assert`s get stripped away in release builds. To make sure the checks in tests still run in release builds, this switches to `expectTrue`/`expectFalse` instead of `assert`.

Author: egorzhdan

test/Interop/Cxx/foreign-reference/inheritance.swift

@@ -16,18 +16,18 @@ var TemplatingTestSuite = TestSuite("Foreign references work with templates")
 
 TemplatingTestSuite.test("SubT") {
     let s: SubT = SubT.getSubT()
-    assert(!s.isBase)
+    expectFalse(s.isBase)
     let sc: BaseT = cast(s)
-    assert(!sc.isBase)
+    expectFalse(sc.isBase)
     let sx: BaseT = cxxCast(s)      // should instantiate I to SubT and O to BaseT
-    assert(!sc.isBase)
+    expectFalse(sc.isBase)
 }
 
 TemplatingTestSuite.test("BaseT") {
     let b: BaseT = BaseT.getBaseT()
-    assert(b.isBase)
+    expectTrue(b.isBase)
     let bc: BaseT = cxxCast(b)      // should instantiate I and O both to BaseT
-    assert(bc.isBase)
+    expectTrue(bc.isBase)
 }
 
 runAllTests()