Make shtest-format.py CHECK lines more flexible

The test sometimes fails on Windows due to a warning emitted by bash about not
being able to find the /tmp directory causing this test to randomly fail. This
update makes the test more flexible to account for this possibility and should
hopefully make it more reliable.

Reviewed By: probinson

Differential Revision: https://reviews.llvm.org/D118691

Author: dyung

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

@@ -17,7 +17,8 @@
 # CHECK-NEXT: line 2: failed test output on stdout
 # CHECK: Command Output (stderr):
 # CHECK-NEXT: --
-# CHECK-NEXT: cat{{(_64)?(\.exe)?}}: {{cannot open does-not-exist|does-not-exist: No such file or directory}}
+# CHECK-NOT: --
+# 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