[AIX] Fix AIX failures due to switch to internal shell (#160566)

This PR fixes a bunch of failures on AIX that occurred due to the switch
to lit internal shell by default. The failures deal with the following:
1. unset not being supported by lit internal shell
2. A bug with echo -n on AIX when there are multiple pipes in the RUN
command
3. ulimit test case not supported on AIX due to the -v option
4. platform specific error message for missing file

Author: madanial0

tests/shtest-readfile-external.py

@@ -11,7 +11,7 @@
 
 # CHECK-LABEL: FAIL: shtest-readfile :: file-does-not-exist.txt ({{[^)]*}})
 # CHECK: echo $(cat /file/does/not/exist) && test -e /file/does/not/exist {{.*}}
-# CHECK: cat: /file/does/not/exist: No such file or directory
+# CHECK: {{.*}}cat{{.*}}/file/does/not/exist{{.*}}
 
 # CHECK-LABEL: FAIL: shtest-readfile :: relative-paths.txt ({{[^)]*}})
 # CHECK: echo $(cat rel_path_test_folder/test_file) && test -e rel_path_test_folder/test_file {{.*}}

tests/shtest-ulimit-nondarwin.py

@@ -2,7 +2,7 @@
 
 # ulimit does not work on non-POSIX platforms.
 # These tests are specific to options that Darwin does not support.
-# UNSUPPORTED: system-windows, system-darwin
+# UNSUPPORTED: system-windows, system-darwin, system-aix
 
 # RUN: not %{lit} -a -v %{inputs}/shtest-ulimit-nondarwin | FileCheck %s