Use exact IPv4/IPv6 address in test_run:cmd()

When test-run's inspector listens on a IPv4 port and some other process
listens on a IPv6 port of the same number, test_run:cmd() could connect
to this process rather then the inspector, because 'localhost' hostname
was used to connect.

Now test-run passes actual IP address of the inspector via an
environment variable and test_run:cmd() uses it to avoid the ambiguity
that is described above.

The problem was found during many parallel runs of app/socket.test.lua
test from tarantool, see case 4 in [1] (untitled, see right under case
3). This test starts many TCP servers and in rare cases a server port
may be the same as inspector's one.

There are two details that increase probability of the problem:

* It seems test-run's inspector always listens on IPv4 address, because
  it is configured with 'localhost' hostname and
  gevent.server.StreamServer implementation always chooses IPv4 for
  'localhost' (see [2], _parse_address() function).

* socket.tcp_connect() typically tries to connect to an IPv6 address
  first and only then to an IPv4 one, because getaddrinfo() returns
  addresses in this order according to RFC 3484 (see [3], section 2.1
  'Policy table'). This however can be configured in /etc/gai.conf.

[1]: https://lists.tarantool.org/pipermail/tarantool-patches/2019-December/013321.html
[2]: https://github.com/gevent/gevent/blob/ba3b15de7eb568fa09fdf1a9d28241288163ba67/src/gevent/baseserver.py#L406-L409
[3]: https://tools.ietf.org/rfc/rfc3484.txt

lib/inspector.py

@@ -55,10 +55,12 @@ def __init__(self, host, port):
 
     def start(self):
         super(TarantoolInspector, self).start()
-        os.environ['INSPECTOR'] = str(self.server_port)
+        os.environ['INSPECTOR_HOST'] = str(self.server_host)
+        os.environ['INSPECTOR_PORT'] = str(self.server_port)
 
     def stop(self):
-        del os.environ['INSPECTOR']
+        del os.environ['INSPECTOR_HOST']
+        del os.environ['INSPECTOR_PORT']
 
     def set_parser(self, parser):
         self.parser = parser

test_run.lua

@@ -458,8 +458,8 @@ local inspector_methods = {
 local function inspector_new(host, port)
     local inspector = {}
 
-    inspector.host = host or 'localhost'
-    inspector.port = port or tonumber(os.getenv('INSPECTOR'))
+    inspector.host = host or os.getenv('INSPECTOR_HOST') or 'localhost'
+    inspector.port = port or tonumber(os.getenv('INSPECTOR_PORT'))
     if inspector.port == nil then
         error('Inspector not started')
     end