[compiler-rt] Update fuzzer/focus-function.test to not require shell. (#157967)

Replace sub-shell for-loops with python calls so this test can run in
the lit internal shell. This is part of our work migrating to use the
internal shell as the default for lit tests.

compiler-rt/test/fuzzer/focus-function.test

diff --git a/compiler-rt/test/fuzzer/focus-function.test b/compiler-rt/test/fuzzer/focus-function.test
index aa15692..717af0e 100644
--- a/compiler-rt/test/fuzzer/focus-function.test
+++ b/compiler-rt/test/fuzzer/focus-function.test
@@ -1,8 +1,7 @@
 # Tests -focus_function
 #
 # TODO: don't require linux.
-# Requires full shell support for the `for` loop syntax.
-# REQUIRES: shell, linux
+# REQUIRES: linux
 UNSUPPORTED: target=aarch64{{.*}}
 
 RUN: %cpp_compiler %S/OnlySomeBytesTest.cpp -o %t-exe
@@ -22,9 +21,9 @@
 RUN: rm -rf %t-corpus
 RUN: mkdir %t-corpus
 # ABC triggers the focus function, others don't.
-RUN: echo ABC$(for((i=0;i<2048;i++)); do echo -n x; done) > %t-corpus/ABC
-RUN: echo AXY$(for((i=0;i<2048;i++)); do echo -n x; done) > %t-corpus/AXY
-RUN: echo ABX$(for((i=0;i<2048;i++)); do echo -n x; done) > %t-corpus/ABX
+RUN: %python -c 'print("ABC" + "x" * 2048)' > %t-corpus/ABC
+RUN: %python -c 'print("AXY" + "x" * 2048)' > %t-corpus/AXY
+RUN: %python -c 'print("ABX" + "x" * 2048)' > %t-corpus/ABX
 
 RUN: %run %t-exe -runs=10000 -focus_function=f0 %t-corpus 2>&1 | FileCheck %s --check-prefix=CORPUS_1_3
 CORPUS_1_3: INFO: 1/3 inputs touch the focus function