Add test failure threshold to pester.sh

Set to 14 because of non-tty and random process failures.

pester.sh

@@ -1,3 +1,10 @@
-./bin/powershell -c "Invoke-Pester test/powershell/$1 -OutputFile pester-tests.xml -OutputFormat NUnitXml"
+#!/usr/bin/env bash
+
+./bin/powershell -c "Invoke-Pester test/powershell/$1 -OutputFile pester-tests.xml -OutputFormat NUnitXml -EnableExit"
+failed_tests=$?
+
 # XML files are not executable
 chmod -x pester-tests.xml
+
+# Exit with failure if number of failed tests exceeds threshold
+[ ! $failed_tests -gt 14 ]