[gcmole] Propagate errors during test run

The bot that runs gcmole was failing before
https://crrev.com/c/1789707 because the test file was missing.
It returned with exit status 0 anyway though. After fixing the
original fault, this CL ensures that the gcmole tests also
trigger an error on the bot(s) if they fail.

R=mstarzinger@chromium.org
CC=​​mslekova@chromium.org

Change-Id: I29ae40301062baadfcd38b26c336c5749924b0d4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1789702
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63612}

tools/gcmole/gcmole.lua

@@ -467,6 +467,10 @@ end
 
 local function TestRun()
    local errors, output = SafeCheckCorrectnessForArch('x64', true)
+   if not errors then
+      log("** Test file should produce errors, but none were found.")
+      return false
+   end
 
    local filename = "tools/gcmole/test-expectations.txt"
    local exp_file = assert(io.open(filename), "failed to open test expectations file")
@@ -474,18 +478,18 @@ local function TestRun()
 
    if output ~= expectations then
       log("** Output mismatch from running tests. Please run them manually.")
-   else
-      log("** Tests ran successfully")
+      return false
    end
-end
 
-TestRun()
+   log("** Tests ran successfully")
+   return true
+end
 
-local errors = false
+local errors = not TestRun()
 
 for _, arch in ipairs(ARCHS) do
    if not ARCHITECTURES[arch] then
-      error ("Unknown arch: " .. arch)
+      error("Unknown arch: " .. arch)
    end
 
    errors = SafeCheckCorrectnessForArch(arch, false) or errors