[lit][tests][AIX] Update expected form of diagnostic messages; use `not` to normalize non-zero exit values

[hubert.reinterpretcast]

Summary

Various tests in the lit testing suite expect specific return codes and forms of diagnostic message from utility programs. As per POSIX.1-2017 XCU Section 1.4, Utility Description Defaults, "[the] format of diagnostic messages for most utilities is unspecified". The STDERR subsections of the cat and wc utilities merely indicate that "[the] standard error shall be used only for diagnostic messages". The corresponding EXIT STATUS subsections merely indicate, with regard to errors, an exit value of >0.

The affected tests are updated to accept the applicable diagnostic message as produced by the utilities on AIX. The exit value is normalized using not as necessary.

[xingxue]

LGTM.

utils/lit/tests/Inputs/shtest-output-printing/basic.txt

 # RUN: true
 # RUN: echo hi
-# RUN: wc missing-file &> %t.out
+# RUN: not not wc missing-file &> %t.out

utils/lit/tests/shtest-format.py

 # CHECK: FAIL: shtest-format :: external_shell/fail.txt
 # CHECK-NEXT: *** TEST 'shtest-format :: external_shell/fail.txt' FAILED ***
 # CHECK: Command Output (stdout):
 # CHECK-NEXT: --
 # CHECK-NEXT: line 1: failed test output on stdout
 # CHECK-NEXT: line 2: failed test output on stdout
 # CHECK: Command Output (stderr):
 # CHECK-NEXT: --
-# CHECK-NEXT: cat{{(\.exe)?}}: does-not-exist: No such file or directory
+# CHECK-NEXT: cat{{(\.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
 # CHECK-NEXT: *** TEST 'shtest-format :: external_shell/fail_with_bad_encoding.txt' FAILED ***
 # CHECK: Command Output (stdout):
 # CHECK-NEXT: --
 # CHECK-NEXT: a line with bad encoding:
 # CHECK: --

utils/lit/tests/shtest-output-printing.py

 # CHECK-NEXT: $ ":" "RUN: at line 1"
 # CHECK-NEXT: $ "true"
 # CHECK-NEXT: $ ":" "RUN: at line 2"
 # CHECK-NEXT: $ "echo" "hi"
 # CHECK-NEXT: # command output:
 # CHECK-NEXT: hi
 #
 # CHECK:      $ ":" "RUN: at line 3"
-# CHECK-NEXT: $ "wc" "missing-file"
+# CHECK-NEXT: $ "not" "not" "wc" "missing-file"
 # CHECK-NEXT: # redirected output from '{{.*(/|\\\\)}}basic.txt.tmp.out':
-# CHECK-NEXT: missing-file{{.*}} No such file or directory
+# CHECK-NEXT: {{cannot open missing-file|missing-file.* No such file or directory}}
 # CHECK:      note: command had no output on stdout or stderr
 # CHECK-NEXT: error: command failed with exit status: 1