tests: make calls to KcovTestCase.do more strict

Update the KcovTestCase.do method signature, so that kcovKcov
is both a positional or keyword parameter, and the timeout is a keyword
only parameter.

I was planning to kame kcovKcov a positional only parameter, but the
`bash_linux_only.bash_exit_before_child` test used it as a keyword
parameter.

This change codifies the current usage.

tests/tools/testbase.py

@@ -75,7 +75,7 @@ def doShell(self, cmdline):
 
         return rv, output
 
-    def do(self, cmdline, kcovKcov=True, timeout=default_timeout):
+    def do(self, cmdline, /, kcovKcov=True, *, timeout=default_timeout):
         extra = ""
         if (
             kcovKcov