test428: re-enable for Windows

[vszakats]

The unexplained error in AppVeyor CI tests are not hit in CI after
moving those tests to GHA. Re-enable to run this test on Windows.

Revisit if the error is seen again on Windows.

Errors seen earlier in AppVeyor CI:
https://ci.appveyor.com/project/curlorg/curl/builds/49120834
https://ci.appveyor.com/project/curlorg/curl/builds/49123802 (with debug lines)

In these jobs:
CMake, VS2010, Debug, x64, no SSL, Static
CMake, mingw-w64, gcc 7, Debug, x64, Schannel, Static, Unicode
CMake, mingw-w64, gcc 9, Debug, x64, Schannel, Static, Unity
CMake, mingw-w64, gcc 6, Debug, x86, Schannel, Static

Test log from the 'gcc 9` job above (with debug lines):

 test 0428...[Expand environment variables within config file]

  428: protocol FAILED!
  There was no content at all in the file log/server.input.
  Server glitch? Total curl failure? Returned: 26
 == Contents of files in the log/ dir after test 428
 === Start of file cmd
  --variable %FUNVALUE
  --variable %VALUE2
  --variable %BLANK
  --variable %curl_NOT_SET=default
  --expand-data 1{{FUNVALUE}}2{{VALUE2}}3{{curl_NOT_SET}}4{{BLANK}}5\{{verbatim}}6{{not.good}}7{{}}
 === End of file cmd
 === Start of file commands.log
  ../src/curl.exe --output log/curl428.out  --include --trace-ascii log/trace428 --trace-time http://127.0.0.1:1593/428 -K log/cmd > log/stdout428 2> log/stderr428
 === End of file commands.log
 === Start of file server.cmd
  Testnum 428
 === End of file server.cmd
 === Start of file stderr428
  getenv of 'FUNVALUE' returned 0xee65d2
  getenv of 'VALUE2' returned 0xee7a42
  getenv of 'BLANK' returned (nil)
  curl: Variable 'BLANK' import fail, not set
  curl: log/cmd:3: '--variable' variable expansion failure
  curl: cannot read config from 'log/cmd'
  curl: option -K: error encountered when reading a file
  curl: try 'curl --help' for more information
 === End of file stderr428

Env comparison:
Fail: https://ci.appveyor.com/project/curlorg/curl/builds/49123802/job/2a4w7i21npys9pd3

-- curl version=[8.6.1-DEV]
-- The C compiler identification is GNU 9.1.0
-- Found Perl: C:/msys64/usr/bin/perl.exe (found version "5.30.0").
-- Found _WIN32_WINNT=0x0601
* curl 8.6.1-DEV (Windows).
* libcurl/8.6.1-DEV Schannel zlib/1.2.11
* Features: alt-svc AsynchDNS Debug HSTS HTTPS-proxy IPv6 Kerberos Largefile libz NTLM SPNEGO SSL SSPI threadsafe TrackMemory UnixSockets
* Disabled: xattr
* System: MSYS_NT-10.0-14393 APPVYR-WIN 3.0.7-338.x86_64 2019-07-11 10:58 UTC x86_64 Msys

OK (this PR): https://github.com/curl/curl/actions/runs/16439564668/job/46456976494

-- curl version=[8.15.1-DEV]
-- The C compiler identification is GNU 9.5.0
-- Found Perl: C:/msys64/usr/bin/perl.exe (found version "5.38.4")
-- Found _WIN32_WINNT=0x0601
* curl 8.15.1-DEV (Windows).
* libcurl/8.15.1-DEV Schannel libpsl/0.21.5
* Protocols: dict file ftp ftps gopher gophers http https imap imaps ipfs ipns ldap ldaps mqtt pop3 pop3s rtsp smb smbs smtp smtps telnet tftp ws wss
* Features: alt-svc AsynchDNS Debug HSTS HTTPS-proxy IPv6 Kerberos Largefile NTLM PSL SPNEGO SSL SSPI threadsafe TrackMemory UnixSockets
* Disabled: xattr, win32-ca-search-safe, override-dns
* System: MINGW64_NT-10.0-20348 runnervm51nrn 3.6.3-ab81aae6.x86_64 2025-07-01 18:20 UTC x86_64 Msys

Follow-up to 7cf8414
Ref: #12862 (comment)
Ref: 0f0edc2 #12862

[vszakats]

Most of the effort went to tracing back the original PR
and find the CI logs with the Windows issue. This PR
includes the links and log snippet, to have it documented.
I couldn't make any conclusion beyond that, but may help
if the issue resurfaces in the future.

[MarcelRaad]

The failure appears in both of my autobuilds now:
https://curl.se/dev/log.cgi?id=20250820041228-707387#prob1

The first error is:

curl: Variable 'BLANK' import fail, not set

I haven't had time to debug this so far, unfortunately.

[vszakats]

The failing job is using Perl v5.32.0 (/usr/bin/perl) in case it makes a difference:
https://curl.se/dev/log.cgi?id=20250801033352-2611825