PP: Fix #1104: Missing check for #if overflow.

Also, rationalized this to generally make it safer and more readable.
It could use a more modern approach, at some point...
diff --git a/gtests/AST.FromFile.cpp b/gtests/AST.FromFile.cpp
index fbbf81b..b963337 100644
--- a/gtests/AST.FromFile.cpp
+++ b/gtests/AST.FromFile.cpp
@@ -82,6 +82,7 @@
         "cppBad.vert",
         "cppBad2.vert",
         "cppComplexExpr.vert",
+        "cppDeepNest.frag",
         "badChars.frag",
         "pointCoord.frag",
         "array.frag",