str_cat_test: Avoid using invalid enum values

Invalid enum values are undefined behavior. This change
makes the enum backed by an integer for testing.

Our UBSAN tests were missing `-fno-sanitize-recover`, which
means UBSAN logs a warning, but the program continues,
causing the test not to fail.

`-fno-sanitize-recover` will be added once all errors are fixed.

PiperOrigin-RevId: 733450563
Change-Id: I0644af2ecb9ca6def5fa0c9c948d76dda54af97f

Author: derekmauro

absl/strings/str_cat_test.cc

@@ -78,7 +78,7 @@ TEST(StrCat, Ints) {
 TEST(StrCat, Enums) {
   enum SmallNumbers { One = 1, Ten = 10 } e = Ten;
   EXPECT_EQ("10", absl::StrCat(e));
-  EXPECT_EQ("-5", absl::StrCat(SmallNumbers(-5)));
+  EXPECT_EQ("1", absl::StrCat(One));
 
   enum class Option { Boxers = 1, Briefs = -1 };