tests: Add `rhg` and `no-rhg` for #require and #if in .t files

tests: Add `rhg` and `no-rhg` for #require and #if in .t files

With this we can allow some small differences in expected output
between Rust-based and Python-based code paths.

Differential Revision: https://phab.mercurial-scm.org/D10096

--- a/tests/hghave.py	Tue Mar 02 22:42:10 2021 +0100
+++ b/tests/hghave.py	Wed Mar 03 18:00:54 2021 +0100
@@ -188,6 +188,11 @@
     return 'CHGHG' in os.environ
 
 
+@check("rhg", "running with rhg as 'hg'")
+def has_rhg():
+    return 'RHG_INSTALLED_AS_HG' in os.environ
+
+
 @check("cvs", "cvs client/server")
 def has_cvs():
     re = br'Concurrent Versions System.*?server'

--- a/tests/run-tests.py	Tue Mar 02 22:42:10 2021 +0100
+++ b/tests/run-tests.py	Wed Mar 03 18:00:54 2021 +0100
@@ -3134,6 +3134,8 @@
         # configure fallback and replace "hg" command by "rhg"
         rhgbindir = self._bindir
         if self.options.rhg or self.options.with_rhg:
+            # Affects hghave.py
+            osenvironb[b'RHG_INSTALLED_AS_HG'] = b'1'
             # Affects configuration. Alternatives would be setting configuration through
             # `$HGRCPATH` but some tests override that, or changing `_hgcommand` to include
             # `--config` but that disrupts tests that print command lines and check expected

--- a/tests/test-rhg.t	Tue Mar 02 22:42:10 2021 +0100
+++ b/tests/test-rhg.t	Wed Mar 03 18:00:54 2021 +0100
@@ -1,15 +1,4 @@
-#require rust
-
-Define an rhg function that will only run if rhg exists
-  $ RHG="$RUNTESTDIR/../rust/target/release/rhg"
-  $ rhg() {
-  > if [ -f "$RHG" ]; then
-  >   "$RHG" "$@"
-  > else
-  >   echo "skipped: Cannot find rhg. Try to run cargo build in rust/rhg."
-  >   exit 80
-  > fi
-  > }
+#require rhg
 
   $ NO_FALLBACK="env RHG_ON_UNSUPPORTED=abort"
 
@@ -169,8 +158,8 @@
   unsupported feature: `rhg cat` without `--rev` / `-r`
   [252]
 
-  $ rhg cat original --config rhg.fallback-executable="$RHG"
-  Blocking recursive fallback. The 'rhg.fallback-executable = */rust/target/release/rhg' config points to `rhg` itself. (glob)
+  $ rhg cat original --config rhg.fallback-executable=rhg
+  Blocking recursive fallback. The 'rhg.fallback-executable = rhg' config points to `rhg` itself.
   unsupported feature: `rhg cat` without `--rev` / `-r`
   [252]