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)
@@ -695,33 +694,13 @@ local function check_child_err(rd)
     end
   end
   local res = sc.read(rd, 2)
-  if #res ~= 2 then
+  if #res == 2 then
-    local error
+    local err = ''
-    if #trace == 0 then
+    if res ~= '+\n' then
-      error = '\nTest crashed, no trace available\n'
-    else
-      error = '\nTest crashed, trace:\n' .. tracehelp
-      for i = 1, #trace do
-        error = error .. trace[i]
-      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
@@ -729,8 +708,30 @@ local function check_child_err(rd)
         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 err ~= '' then
       assert.just_fail(err)
     end
+  end
+end
 
 local function itp_parent(rd, pid, allow_failure)
   local err, emsg = pcall(check_child_err, rd)