[MachO] Fix test on llvm-x86_64-debian-dylib builder

Make the `--implicit-check-not` account for the file extension as well.
This will still fail if we ever have a builder with `.dylib` in its
name, and we probably want a more robust solution, but this addresses
the immediate issue.

Fixes https://github.com/llvm/llvm-project/issues/75850
Fixes https://github.com/llvm/llvm-project/issues/75910

GitOrigin-RevId: c77c3663dbea07264ba760bc17b913cebd7986ba
diff --git a/test/MachO/flat-namespace-dysyms.s b/test/MachO/flat-namespace-dysyms.s
index 2ac3b29..8d06932 100644
--- a/test/MachO/flat-namespace-dysyms.s
+++ b/test/MachO/flat-namespace-dysyms.s
@@ -32,7 +32,7 @@
 # RUN: cd %t
 # RUN: tar -tf repro.tar | FileCheck -DPATH='%:t.dir' --check-prefix=REPRO %s
 # RUN: tar -xf repro.tar repro/response.txt
-# RUN: FileCheck --implicit-check-not=dylib --check-prefix=RESPONSE %s \
+# RUN: FileCheck --implicit-check-not=.dylib --check-prefix=RESPONSE %s \
 # RUN:     < %t/repro/response.txt
 
 # HEADERBITS-NOT: NOUNDEFS