run-tests: use more explicit signaling for `chg`

Using a dedicated variable is clearer and less fragile. It cannot hurt.

Differential Revision: https://phab.mercurial-scm.org/D11051

tests/hghave.py

@@ -191,7 +191,7 b' def has_bzr():'
 
 @check("chg", "running with chg")
 def has_chg():
-    return 'CHGHG' in os.environ
+    return 'CHG_INSTALLED_AS_HG' in os.environ
 
 
 @check("rhg", "running with rhg as 'hg'")

tests/run-tests.py

@@ -3165,9 +3165,11 b' class TestRunner(object):'
         # set CHGHG, then replace "hg" command by "chg"
         chgbindir = self._bindir
         if self.options.chg or self.options.with_chg:
+            osenvironb[b'CHG_INSTALLED_AS_HG'] = b'1'
             osenvironb[b'CHGHG'] = real_hg
         else:
-            osenvironb.pop(b'CHGHG', None)  # drop flag for hghave
+            # drop flag for hghave
+            osenvironb.pop(b'CHG_INSTALLED_AS_HG', None)
         if self.options.chg:
             self._hgcommand = b'chg'
         elif self.options.with_chg: