Use FileCheck in new clang-tidy/infrastructure/config-files.cpp tests

The test was failing spuriously on some machines [1]. Converting from
grep to FileCheck makes it easier to see what's happening, and also grep
is discouraged in tests [2].

Also drop the negative test: checking for the expected output is better.

 1. https://crbug.com/338535754
 2. https://llvm.org/docs/TestingGuide.html#:~:text=.%20%5BThe-,usage%20of%20grep,-in%20RUN%20lines

Author: zmodem

clang-tools-extra/test/clang-tidy/infrastructure/config-files.cpp

@@ -66,9 +66,5 @@
 // RUN: clang-tidy --checks="-*,readability-identifier-naming" --dump-config %S/Inputs/config-files/- -- | grep "readability-identifier-naming\." | sort --check
 
 // Dumped config does not overflow for unsigned options
-// RUN: clang-tidy --dump-config \
-// RUN: --checks="-*,misc-throw-by-value-catch-by-reference" \
-// RUN: -- | grep -v -q "misc-throw-by-value-catch-by-reference.MaxSize: '-1'"
-
-// RUN: clang-tidy --dump-config %S/Inputs/config-files/5/- \
-// RUN: -- | grep -q "misc-throw-by-value-catch-by-reference.MaxSize: '1152921504606846976'"
+// RUN: clang-tidy --dump-config %S/Inputs/config-files/5/- -- | FileCheck %s -check-prefix=CHECK-OVERFLOW
+// CHECK-OVERFLOW: misc-throw-by-value-catch-by-reference.MaxSize: '1152921504606846976'