[BigArchive][NFC] Enhance test cases introduced in D138986

[lkail]

Summary

Per @jhenderson 's comments, update test cases.

[jhenderson]

Looks good, with one optional suggestion.

llvm/test/Object/archive-big-read-empty-with-freelist.test

2

If you'd like to verify that the output is empty, you can add | count 0 to the end of this line (see other examples in the lit tests again).

[lkail]

Changes are included in https://reviews.llvm.org/D138986.

llvm/test/Object/archive-big-malformed-first-member.test

-# Test reading an empty archive with first member's offset is not zero.
+## Test reading an empty archive with first member's offset is not zero.
 # RUN: echo    "<bigaf>" > %t.a
 # RUN: echo -n "0                   0                   0                   128                 0                   0                   " >> %t.a
-# RUN: not llvm-ar tv %t.a 2>&1 | grep 'truncated or malformed archive'
+# RUN: not llvm-ar tv %t.a 2>&1 | FileCheck %s
 # RUN: echo    "<bigaf>" > %t.a
 # RUN: echo -n "0                   0                   0                   28                  0                   0                   " >> %t.a
-# RUN: not llvm-ar tv %t.a 2>&1 | grep 'truncated or malformed archive'
+# RUN: not llvm-ar tv %t.a 2>&1 | FileCheck %s
+# CHECK: truncated or malformed archive

llvm/test/Object/archive-big-read-empty-with-freelist.test

-# Test reading an empty archive with free list in it.
-# RUN: llvm-ar tv %p/Inputs/aix-empty-big-archive-with-freelist.a 2>&1 \
-# RUN:   | not grep 'truncated or malformed archive'
+## Test reading an empty archive with free list in it, should exit normally.
+# RUN: llvm-ar tv %p/Inputs/aix-empty-big-archive-with-freelist.a 2>&1