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
<stripfile>
$_ = '' if((($_ !~ /tool_paramhlp/) and ($_ !~ /tool_cfgable/)) || ($_ =~ /free\(\(nil\)\)/))
s/:\d+.*//
-s:^(MEM )(.*/)(.*):$1$3:
+s:^(MEM )(.*[/\\])(.*):$1$3:
</stripfile>
</verify>