Ensure the "kcov-kcov" directory uses a fixed path

In ci.yml, codecov assumes that the output directory is
"/tmp/kcov-kcov".

Don't assume that `outbase` is always "/tmp", and instead use the
"/tmp/kcov-kcov" path.

tests/tools/testbase.py

@@ -66,8 +66,7 @@ def do(self, cmdline, kcovKcov=True, timeout=None, kill=False):
             extra = (
                 kcov
                 + " --include-pattern=kcov --exclude-pattern=helper.cc,library.cc,html-data-files.cc "
-                + outbase
-                + "/kcov-kcov "
+                + "/tmp/kcov-kcov "
             )
 
         cmdline = extra + cmdline