unittests: Do gc after reporting error, not before

Reason: test may contain cleanup at the endwhich is needed for GC to work 
properly, but is not done if test fails. With collectgarbage() in former 
position it would crash when collecting garbage.

test/unit/helpers.lua

@@ -650,8 +650,6 @@ local function itp_child(wr, func)
     collectgarbage('stop')
     child_sethook(wr)
     err, emsg = pcall(func)
-    collectgarbage('restart')
-    collectgarbage()
     debug.sethook()
   end
   emsg = tostring(emsg)
@@ -662,14 +660,15 @@ local function itp_child(wr, func)
     end
     sc.write(wr, ('-\n%05u\n%s'):format(#emsg, emsg))
     deinit()
-    sc.close(wr)
-    sc.exit(1)
   else
     sc.write(wr, '+\n')
     deinit()
-    sc.close(wr)
-    sc.exit(0)
   end
+  collectgarbage('restart')
+  collectgarbage()
+  sc.write(wr, '$\n')
+  sc.close(wr)
+  sc.exit(err and 0 or 1)
 end
 
 local function check_child_err(rd)
@@ -695,41 +694,43 @@ local function check_child_err(rd)
     end
   end
   local res = sc.read(rd, 2)
-  if #res ~= 2 then
-    local error
-    if #trace == 0 then
-      error = '\nTest crashed, no trace available\n'
-    else
-      error = '\nTest crashed, trace:\n' .. tracehelp
-      for i = 1, #trace do
-        error = error .. trace[i]
+  if #res == 2 then
+    local err = ''
+    if res ~= '+\n' then
+      eq('-\n', res)
+      local len_s = sc.read(rd, 5)
+      local len = tonumber(len_s)
+      neq(0, len)
+      if os.getenv('NVIM_TEST_TRACE_ON_ERROR') == '1' and #trace ~= 0 then
+        err = '\nTest failed, trace:\n' .. tracehelp
+        for _, traceline in ipairs(trace) do
+          err = err .. traceline
+        end
+      end
+      err = err .. sc.read(rd, len + 1)
+    end
+    local eres = sc.read(rd, 2)
+    if eres ~= '$\n' then
+      if #trace == 0 then
+        err = '\nTest crashed, no trace available\n'
+      else
+        err = '\nTest crashed, trace:\n' .. tracehelp
+        for i = 1, #trace do
+          err = err .. trace[i]
+        end
+      end
+      if not did_traceline then
+        err = err .. '\nNo end of trace occurred'
+      end
+      local cc_err, cc_emsg = pcall(check_cores, Paths.test_luajit_prg, true)
+      if not cc_err then
+        err = err .. '\ncheck_cores failed: ' .. cc_emsg
       end
     end
-    if not did_traceline then
-      error = error .. '\nNo end of trace occurred'
-    end
-    local cc_err, cc_emsg = pcall(check_cores, Paths.test_luajit_prg, true)
-    if not cc_err then
-      error = error .. '\ncheck_cores failed: ' .. cc_emsg
-    end
-    assert.just_fail(error)
-  end
-  if res == '+\n' then
-    return
-  end
-  eq('-\n', res)
-  local len_s = sc.read(rd, 5)
-  local len = tonumber(len_s)
-  neq(0, len)
-  local err = ''
-  if os.getenv('NVIM_TEST_TRACE_ON_ERROR') == '1' and #trace ~= 0 then
-    err = '\nTest failed, trace:\n' .. tracehelp
-    for _, traceline in ipairs(trace) do
-      err = err .. traceline
+    if err ~= '' then
+      assert.just_fail(err)
     end
   end
-  err = err .. sc.read(rd, len + 1)
-  assert.just_fail(err)
 end
 
 local function itp_parent(rd, pid, allow_failure)