[LLDB] Replace file+filecheck with test in test (NFC) (#186875)

The "file" utility may not be installed:
https://green.lab.llvm.org/job/llvm.org/view/LLDB/job/lldb-ubuntu-aarch64/job/main/109/consoleText

lldb/test/Shell/Diagnostics/TestDump.test

@@ -5,11 +5,9 @@
 # RUN: rm -rf %t.existing
 # RUN: mkdir -p %t.existing
 # RUN: %lldb -o 'diagnostics dump -d %t.existing'
-# RUN: file %t.existing | FileCheck %s
+# RUN: test -d %t.existing
 
 # Dump to a non-existing directory.
 # RUN: rm -rf %t.nonexisting
 # RUN: %lldb -o 'diagnostics dump -d %t.nonexisting'
-# RUN: file %t.nonexisting | FileCheck %s
-
-# CHECK: directory
+# RUN: test -d %t.nonexisting