mercurial: move tests to passthru.tests

They're very expensive to run, especially if you don't have that many
cores, and can sometimes be a bit flaky (it looks like their CI doesn't
run things under the same constraints as we tend to).

Move them to a separate derivation, and make them test the actual
installed output rather than the local copy.

pkgs/applications/version-management/mercurial/default.nix

@@ -7,6 +7,7 @@
 , highlightSupport ? fullBuild
 , ApplicationServices
 # test dependencies
+, runCommand
 , unzip
 , which
 , sqlite
@@ -39,15 +40,6 @@ let
     } else null;
     cargoRoot = if rustSupport then "rust" else null;
 
-    postPatch = ''
-      patchShebangs .
-
-      for f in **/*.{py,c,t}; do
-        # not only used in shebangs
-        substituteAllInPlace "$f" '/bin/sh' '${stdenv.shell}'
-      done
-    '';
-
     propagatedBuildInputs = lib.optional re2Support fb-re2
       ++ lib.optional gitSupport pygit2
       ++ lib.optional highlightSupport pygments;
@@ -63,31 +55,6 @@ let
     makeFlags = [ "PREFIX=$(out)" ]
       ++ lib.optional rustSupport "PURE=--rust";
 
-    doCheck = stdenv.isLinux;  # tests seem unstable on Darwin
-    checkInputs = [
-      unzip
-      which
-      sqlite
-      git
-      gnupg
-    ];
-    SSL_CERT_FILE = "${cacert}/etc/ssl/certs/ca-bundle.crt";  # needed for git
-    checkPhase = ''
-      cat << EOF > tests/blacklists/nix
-      # tests enforcing "/usr/bin/env" shebangs, which are patched for nix
-      test-run-tests.t
-      test-check-shbang.t
-
-      # unstable experimental/unsupported features
-      # https://bz.mercurial-scm.org/show_bug.cgi?id=6633#c1
-      test-git-interop.t
-      EOF
-
-      # extended timeout necessary for tests to pass on the busy CI workers
-      export HGTESTFLAGS="--blacklist blacklists/nix --timeout 1800"
-      make check
-    '';
-
     postInstall = (lib.optionalString guiSupport ''
       mkdir -p $out/etc/mercurial
       cp contrib/hgk $out/bin
@@ -115,7 +82,9 @@ let
         --zsh contrib/zsh_completion
     '';
 
-    passthru.tests = {};
+    passthru.tests = {
+      mercurial-tests = makeTests { flags = "--with-hg=$MERCURIAL_BASE/bin/hg"; };
+    };
 
     meta = with lib; {
       description = "A fast, lightweight SCM system for very large distributed projects";
@@ -127,6 +96,67 @@ let
       platforms = platforms.unix;
     };
   };
+
+  makeTests = { mercurial ? self, nameSuffix ? "", flags ? "" }: runCommand "${mercurial.pname}${nameSuffix}-tests" {
+    inherit (mercurial) src;
+
+    SSL_CERT_FILE = "${cacert}/etc/ssl/certs/ca-bundle.crt";  # needed for git
+    MERCURIAL_BASE = mercurial;
+    nativeBuildInputs = [
+      python
+      unzip
+      which
+      sqlite
+      git
+      gnupg
+    ];
+
+    postPatch = ''
+      patchShebangs .
+
+      for f in **/*.{py,c,t}; do
+        # not only used in shebangs
+        substituteAllInPlace "$f" '/bin/sh' '${stdenv.shell}'
+      done
+
+      for f in **/*.t; do
+        substituteInPlace 2>/dev/null "$f" \
+          --replace '*/hg:' '*/*hg*:' \${/* paths emitted by our wrapped hg look like ..hg-wrapped-wrapped */""}
+          --replace '"$PYTHON" "$BINDIR"/hg' '"$BINDIR"/hg' ${/* 'hg' is a wrapper; don't run using python directly */""}
+      done
+    '';
+
+    # This runs Mercurial _a lot_ of times.
+    requiredSystemFeatures = [ "big-parallel" ];
+
+    # Don't run tests if not-Linux or if cross-compiling.
+    meta.broken = !stdenv.hostPlatform.isLinux || stdenv.buildPlatform != stdenv.hostPlatform;
+  } ''
+    addToSearchPathWithCustomDelimiter : PYTHONPATH "${mercurial}/${python.sitePackages}"
+
+    unpackPhase
+    cd "$sourceRoot"
+    patchPhase
+
+    cat << EOF > tests/blacklists/nix
+    # tests enforcing "/usr/bin/env" shebangs, which are patched for nix
+    test-run-tests.t
+    test-check-shbang.t
+
+    # unstable experimental/unsupported features
+    # https://bz.mercurial-scm.org/show_bug.cgi?id=6633#c1
+    test-git-interop.t
+
+    # doesn't like the extra setlocale warnings emitted by our bash wrappers
+    test-locale.t
+    EOF
+
+    export HGTEST_REAL_HG="${mercurial}/bin/hg"
+    # extended timeout necessary for tests to pass on the busy CI workers
+    export HGTESTFLAGS="--blacklist blacklists/nix --timeout 1800 -j$NIX_BUILD_CORES ${flags}"
+    make check
+    touch $out
+  '';
 in
   self.overridePythonAttrs (origAttrs: {
     passthru = origAttrs.passthru // rec {