Provide bless_tests.patch for failing tests on AppVeyor

In case any tests are failing with the `--bless` option, we provide the
resulting `git diff` as artifact.  This is particularly helpful for PR
authors who don't have a Windows environment at hand.

Closes GH-7204.

appveyor/test_task.bat

@@ -112,10 +112,16 @@ mkdir c:\tests_tmp
 set TEST_PHP_JUNIT=c:\junit.out.xml
 
 cd "%APPVEYOR_BUILD_FOLDER%"
-nmake test TESTS="%OPCACHE_OPTS% -q --offline --show-diff --show-slow 1000 --set-timeout 120 --temp-source c:\tests_tmp --temp-target c:\tests_tmp %PARALLEL%"
+nmake test TESTS="%OPCACHE_OPTS% -q --offline --show-diff --show-slow 1000 --set-timeout 120 --temp-source c:\tests_tmp --temp-target c:\tests_tmp --bless %PARALLEL%"
 
 set EXIT_CODE=%errorlevel%
 
 appveyor PushArtifact %TEST_PHP_JUNIT%
 
+if %EXIT_CODE% GEQ 1 (
+	git checkout ext\pgsql\tests\config.inc
+	git diff > bless_tests.patch
+	appveyor PushArtifact bless_tests.patch
+)
+
 exit /b %EXIT_CODE%