run-tests: move install.err into test area

Without this, sometimes installerrs generated errors
about no such file. It also did not work well when you
had multiple tests runners running around.

It also did not make sense to pollute the repository test
directory with the log file.

tests/run-tests.py

@@ -2314,7 +2314,7 b' class TestRunner(object):'
         This will also configure hg with the appropriate testing settings.
         """
         vlog("# Performing temporary installation of HG")
-        installerrs = os.path.join(b"tests", b"install.err")
+        installerrs = os.path.join(self._hgtmp, b"install.err")
         compiler = ''
         if self.options.compiler:
             compiler = '--compiler ' + self.options.compiler