tests: scope SL_DEBUG_DISABLE_SYMLINKS to Windows only

Summary:
The `SL_DEBUG_DISABLE_SYMLINKS` was changed to be Windows-only in D71445977.
Update tests to only set it on Windows. `test-no-symlinks.t` now uses `#require
no-symlinks` to cover non-symlink filesystems.

I chose this instead of respecting `SL_DEBUG_DISABLE_SYMLINKS` on non-Windows
platforms because:
- Setting this environment variable on non-Windows can be a footgun for users.
- This environment variable can be deleted once our Windows users are all using
  symlinks. This diff makes it slightly easier to delete.

Reviewed By: muirdm

Differential Revision: D71666114

fbshipit-source-id: 67dd72b4eed768d8d4a9ab43a1a22a4894af59b7

Author: quark-zju

eden/scm/tests/test-no-symlinks.t

@@ -1,11 +1,9 @@
 #modern-config-incompatible
 
-#require no-eden
+#require no-eden no-symlinks
 
 TODO(debugruntest): this test fails on Mac
 #chg-compatible
-  $ export SL_DEBUG_DISABLE_SYMLINKS=1
-
 
 # The following (test) script was used to create the bundle:
 #

eden/scm/tests/test-status.t

@@ -593,7 +593,7 @@ Make sure we expand env vars in ignore file path.
 
   $ cd ..
 
-#if symlink
+#if symlink windows
 Ignore suspiciously modified symlinks.
 
   $ newclientrepo suspicious-symlink

eden/scm/tests/test-symlink-placeholder.t

@@ -1,4 +1,4 @@
-#require symlink no-eden
+#require symlink no-eden windows
 
   $ hg init unix-repo
   $ cd unix-repo

eden/scm/tests/test-symlinks.t

@@ -325,8 +325,10 @@ Issue995: hg copy -A incorrectly handles symbolic links
 
   $ cd ..
 
+#if windows
 Don't treat symlinks as untrackable if symlinks aren't supported.
   $ newclientrepo
   $ ln -s foo bar
   $ SL_DEBUG_DISABLE_SYMLINKS=1 hg status
   ? bar
+#endif