[Fuzzer] make big-file-copy.test work with the internal shell (#168658)

This patch uses several shell features not supported by the internal
shell, such as $? to get the exit code of a command, and exit. This
patch adjusts the test to work with the internal shell by using bash to
run the actual command with a zero exit code to ensure the file is
deleted, and python to propagate the exit code up to lit.

Author: boomanaiden154

compiler-rt/test/fuzzer/big-file-copy.test

@@ -1,6 +1,5 @@
 REQUIRES: darwin
 UNSUPPORTED: ios
 RUN: %cpp_compiler %S/BigFileCopy.cpp -o %t
-RUN: %run %t -runs=1 -rss_limit_mb=4096 2>big-file-out.txt; result=$?
-RUN: %run rm -f big-file.txt big-file-out.txt
-RUN: (exit $result)
+RUN: bash -c '%run %t -runs=1 -rss_limit_mb=4096 2>big-file-out.txt; result=$? \
+RUN:   %run rm -f big-file.txt big-file-out.txt && exit $result'