[AIX] [lit] Fix shtest-format.py to account for behaviour on AIX (#121426)

The changes from #121376 has
broken the ppc64 aix bot:
https://lab.llvm.org/buildbot/#/builders/64/builds/1835. Adjusted the
testcase to account for `cat` behaviour on AIX prior to the changes

Author: madanial0

llvm/utils/lit/tests/shtest-format.py

@@ -18,7 +18,7 @@
 # CHECK: Command Output (stderr):
 # CHECK-NEXT: --
 # CHECK-NOT: --
-# CHECK: cat{{(_64)?(\.exe)?}}: {{.*does-not-exist.*}}: No such file or directory
+# CHECK: cat{{(_64)?(\.exe)?}}: {{(cannot open does-not-exist|.*does-not-exist.*: No such file or directory)}}
 # CHECK: --
 
 # CHECK: FAIL: shtest-format :: external_shell/fail_with_bad_encoding.txt