tests: switch test from '--config ui.editor=~/foo' to HGEDITOR=~/foo

The test is currently not testing anything, the '--config ui.editor=~/path'
argument is apparently of lower priority than the HGEDITOR that run-tests.py is
setting.

Differential Revision: https://phab.mercurial-scm.org/D1809

tests/test-install.t

@@ -98,7 +98,7 path variables are expanded (~ is the sa
 #if execbit
   $ chmod 755 tools/testeditor.exe
 #endif
-  $ hg debuginstall --config ui.editor=~/tools/testeditor.exe
+  $ HGEDITOR="~/tools/testeditor.exe" hg debuginstall
   checking encoding (ascii)...
   checking Python executable (*) (glob)
   checking Python version (*) (glob)
@@ -116,7 +116,7 path variables are expanded (~ is the sa
   checking "re2" regexp engine \((available|missing)\) (re)
   checking templates (*mercurial?templates)... (glob)
   checking default template (*mercurial?templates?map-cmdline.default) (glob)
-  checking commit editor... (* -c "import sys; sys.exit(0)") (glob)
+  checking commit editor... ($TESTTMP/tools/testeditor.exe)
   checking username (test)
   no problems detected