tests: add guard check for suid support

This adds a check for setuid and setgid support and guards a part of a test
requiring this particular filesystem feature.

Such feature is notably missing in the Nix/NixOS build sandbox and was causing
the whole test to fail for the mercurial package.

Differential Revision: https://phab.mercurial-scm.org/D11739

tests/hghave.py

@@ -264,6 +264,13 b' def has_executablebit():'
     return not (new_file_has_exec or exec_flags_cannot_flip)
 
 
+@check("suidbit", "setuid and setgid bit")
+def has_suidbit():
+    if getattr(os, "statvfs", None) is None or getattr(os, "ST_NOSUID") is None:
+        return False
+    return bool(os.statvfs('.').f_flag & os.ST_NOSUID)
+
+
 @check("icasefs", "case insensitive file system")
 def has_icasefs():
     # Stolen from mercurial.util

tests/test-remotefilelog-permissions.t

@@ -33,6 +33,8 b''
 
   $ chmod -R u+w master/.hg/remotefilelogcache
 
+#if suidbit
+
 # Test setting up shared cache with the right permissions
 # (this is hard to test in a cross platform way, so we just make sure nothing
 # crashes)
@@ -44,3 +46,5 b''
   1 files fetched over 1 fetches - (1 misses, 0.00% hit ratio) over * (glob)
   $ ls -ld $CACHEDIR/11
   drwxrws* $TESTTMP/hgcache/11 (glob)
+
+#endif