test: when stripping binaries, ignore case in suppressing "File format not recognized"

The grep -v matches all lowercase, but "file" is captialized; just ignore case so
it's suppressed for either all lowercase or capital File.

test/test-functions

@@ -642,7 +642,7 @@ strip_binaries() {
     ddebug "Strip binaries"
     find "$initdir" -executable -not -path '*/lib/modules/*.ko' -type f | \
         xargs strip --strip-unneeded |& \
-        grep -v 'file format not recognized' | \
+        grep -vi 'file format not recognized' | \
         ddebug
 }