Add --not-clean parameter and dont clean workdir

Now luatest will always clear the server vardir (`/tmp/t` by default)
before running tests. You should be specify a new `--not-clean` parameter
to disable this deletion.

Also luatest does not delete the server workdir in the Server:drop().

Part of tarantool#308

luatest/runner.lua

@@ -12,6 +12,7 @@ local GenericOutput = require('luatest.output.generic')
 local hooks = require('luatest.hooks')
 local loader = require('luatest.loader')
 local pp = require('luatest.pp')
+local Server = require('luatest.server')
 local sorted_pairs = require('luatest.sorted_pairs')
 local TestInstance = require('luatest.test_instance')
 local utils = require('luatest.utils')
@@ -107,6 +108,8 @@ Options:
                           May be repeated to exclude several patterns
                           Make sure you escape magic chars like +? with %
   --coverage:             Use luacov to collect code coverage.
+  --not-clean:            Do not clean the working directory (/tmp/t).
+                          This is cleared by default.
 ]]
 
 function Runner.parse_cmd_line(args)
@@ -170,6 +173,8 @@ function Runner.parse_cmd_line(args)
             result.enable_capture = false
         elseif arg == '--coverage' then
             result.coverage_report = true
+        elseif arg == '--not-clean' then
+            result.not_clean = true
         elseif arg:sub(1,1) == '-' then
             error('Unknown option: ' .. arg)
         elseif arg:find('/') then
@@ -266,6 +271,9 @@ function Runner.mt:initialize()
 end
 
 function Runner.mt:bootstrap()
+    if not self.not_clean then
+        fio.rmtree(Server.vardir)
+    end
     local load_tests = self.load_tests or loader.require_tests
     for _, path in pairs(self.paths) do
         load_tests(path)

luatest/server.lua

@@ -443,8 +443,6 @@ function Server:drop()
     self:stop()
     self:save_artifacts()
 
-    fio.rmtree(self.workdir)
-
     self.instance_id = nil
     self.instance_uuid = nil
 end