Add testsuite tests count to fix Bitbucket pipelines ignoring output

Without this tag, Bitbucket pipelines will pick up the file, but it will 
not visualize it in its interface.

Strangely enough, this does work if you also have other test reports in 
the expected format present, at which point it probably merges it and 
everything is displayed just fine. In other words, this only broke if 
there was just one test report present and it was this one.

Author: __tom__

src/JunitErrorFormatter.php

@@ -89,6 +89,7 @@ public function formatErrors(
             }
 
             $testsuite->setAttribute('name', 'phpstan');
+            $testsuite->setAttribute('tests', (string) $totalErrors);
             $testsuite->setAttribute('failures', (string) $totalErrors);
         }
 

tests/JunitErrorFormatterTest.php

@@ -39,7 +39,7 @@ public function dataFormatterOutputProvider(): Generator
             0,
             '<?xml version="1.0" encoding="UTF-8"?>
 <testsuites name="static analysis">
-  <testsuite failures="1" name="phpstan">
+  <testsuite failures="1" name="phpstan" tests="1">
     <testcase errors="0" failures="1" name="folder with unicode &#x1F603;/file name with &quot;spaces&quot; and unicode &#x1F603;.php:4" tests="1">
       <failure message="Foo" type="error" />
     </testcase>
@@ -55,7 +55,7 @@ public function dataFormatterOutputProvider(): Generator
             1,
             '<?xml version="1.0" encoding="UTF-8"?>
 <testsuites name="static analysis">
-  <testsuite failures="1" name="phpstan">
+  <testsuite failures="1" name="phpstan" tests="1">
     <testcase errors="0" failures="1" name="Generic error" tests="1">
       <failure message="first generic error" type="error" />
     </testcase>
@@ -71,7 +71,7 @@ public function dataFormatterOutputProvider(): Generator
             0,
             '<?xml version="1.0" encoding="UTF-8"?>
 <testsuites name="static analysis">
-  <testsuite failures="4" name="phpstan">
+  <testsuite failures="4" name="phpstan" tests="4">
     <testcase errors="0" failures="1" name="folder with unicode &#x1F603;/file name with &quot;spaces&quot; and unicode &#x1F603;.php:2" tests="1">
       <failure message="Bar" type="error" />
     </testcase>
@@ -96,7 +96,7 @@ public function dataFormatterOutputProvider(): Generator
             2,
             '<?xml version="1.0" encoding="UTF-8"?>
 <testsuites name="static analysis">
-  <testsuite failures="2" name="phpstan">
+  <testsuite failures="2" name="phpstan" tests="2">
     <testcase errors="0" failures="1" name="Generic error" tests="1">
       <failure message="first generic error" type="error" />
     </testcase>
@@ -115,7 +115,7 @@ public function dataFormatterOutputProvider(): Generator
             2,
             '<?xml version="1.0" encoding="UTF-8"?>
 <testsuites name="static analysis">
-  <testsuite failures="6" name="phpstan">
+  <testsuite failures="6" name="phpstan" tests="6">
     <testcase errors="0" failures="1" name="folder with unicode &#x1F603;/file name with &quot;spaces&quot; and unicode &#x1F603;.php:2" tests="1">
       <failure message="Bar" type="error" />
     </testcase>