test96: fix to accept non-unity memdump content with MSVC

test96: fix to accept non-unity memdump content with MSVC

In unity builds the source filename (via `__FILE__`) has no path (or
uses slashes?), while in non-unity ones it does contain backslashes
on Windows, with MSVC. Fix the test to recognize backslashes in the
`stripfile` regexp.

Seen in MSVC jobs in CI:
```diff
-MEM tool_cfgable.c[LF]
-MEM tool_paramhlp.c[LF]
-MEM tool_cfgable.c[LF]
-MEM tool_cfgable.c[LF]
-MEM tool_cfgable.c[LF]
-MEM tool_cfgable.c[LF]
+MEM D:\a\curl\curl\src\tool_cfgable.c[LF]
+MEM D:\a\curl\curl\src\tool_paramhlp.c[LF]
+MEM D:\a\curl\curl\src\tool_cfgable.c[LF]
+MEM D:\a\curl\curl\src\tool_cfgable.c[LF]
+MEM D:\a\curl\curl\src\tool_cfgable.c[LF]
+MEM D:\a\curl\curl\src\tool_cfgable.c[LF]
```
Ref: https://github.com/curl/curl/actions/runs/20408366058/job/58641468316?pr=20061#step:13:303
Ref: https://github.com/curl/curl/actions/runs/20408522070/job/58641826216?pr=20064#step:13:298

Closes #20064