[lit] Update shtest-ulimit.py to explicitly check default limits (llvm#164678)

The recently added ulimit_reset.txt section in shtest-ulimit.py was
failing on some builders if the default file descriptor limit started
with 50. This patch fixes that by explicitly checking that the file
descriptor limit is equal to the default value.

Author: boomanaiden154

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

@@ -5,7 +5,11 @@
 # as well.
 # UNSUPPORTED: system-windows, system-solaris
 
-# RUN: not %{lit} -a -v %{inputs}/shtest-ulimit --order=lexical | FileCheck %s
+# RUN: %{python} %S/Inputs/shtest-ulimit/print_limits.py | grep RLIMIT_NOFILE \
+# RUN:   | sed -n -e 's/.*=//p' | tr -d '\n' > %t.nofile_limit
+
+# RUN: not %{lit} -a -v %{inputs}/shtest-ulimit --order=lexical \
+# RUN:   | FileCheck -DBASE_NOFILE_LIMIT=%{readfile:%t.nofile_limit} %s
 
 # CHECK: -- Testing: 3 tests{{.*}}
 
@@ -18,4 +22,4 @@
 # CHECK: RLIMIT_NOFILE=50
 
 # CHECK-LABEL: FAIL: shtest-ulimit :: ulimit_reset.txt ({{[^)]*}})
-# CHECK-NOT: RLIMIT_NOFILE=50
+# CHECK: RLIMIT_NOFILE=[[BASE_NOFILE_LIMIT]]