run-tests: define the default merge tool through configuration

Using the `HGMERGE` environment variable generates confusion as it overrides
any value set through configuration.
By setting the default value through the default HGRC, the usual overriding
rules apply and it should reduce confusion.

tests/run-tests.py

@@ -1078,7 +1078,6 b' class Test(unittest.TestCase):'
                                                    b'daemon.pids'))
         env["HGEDITOR"] = ('"' + sys.executable + '"'
                            + ' -c "import sys; sys.exit(0)"')
-        env["HGMERGE"] = "internal:merge"
         env["HGUSER"]   = "test"
         env["HGENCODING"] = "ascii"
         env["HGENCODINGMODE"] = "strict"
@@ -1116,6 +1115,7 b' class Test(unittest.TestCase):'
             'EDITOR',
             'GREP_OPTIONS',
             'HG',
+            'HGMERGE',
             'HGPLAIN',
             'HGPLAINEXCEPT',
             'HGPROF',
@@ -1146,6 +1146,7 b' class Test(unittest.TestCase):'
             hgrc.write(b'[ui]\n')
             hgrc.write(b'slash = True\n')
             hgrc.write(b'interactive = False\n')
+            hgrc.write(b'merge = internal:merge\n')
             hgrc.write(b'mergemarkers = detailed\n')
             hgrc.write(b'promptecho = True\n')
             hgrc.write(b'[defaults]\n')

tests/test-basic.t

@@ -9,6 +9,7 b' Create a repository:'
   lfs.usercache=$TESTTMP/.cache/lfs
   ui.slash=True
   ui.interactive=False
+  ui.merge=internal:merge
   ui.mergemarkers=detailed
   ui.promptecho=True
   web.address=localhost

tests/test-commandserver.t

@@ -211,6 +211,7 b' check that local configs for the cached '
   lfs.usercache=$TESTTMP/.cache/lfs
   ui.slash=True
   ui.interactive=False
+  ui.merge=internal:merge
   ui.mergemarkers=detailed
   ui.foo=bar
   ui.nontty=true
@@ -221,6 +222,7 b' check that local configs for the cached '
   *** runcommand -R foo showconfig ui defaults
   ui.slash=True
   ui.interactive=False
+  ui.merge=internal:merge
   ui.mergemarkers=detailed
   ui.nontty=true
 #endif