selftests: Fix runner.sh for non-bash shells

selftests: Fix runner.sh for non-bash shells

Commit 2964f6b816c2 ("selftests: Use ktap helpers for runner.sh") added a
number of bashisms and updated the interpreter specified for the script to
be /bin/bash to reflect this. Unfortunately this does not actually achieve
anything in production since the main way runner.sh is invoked is from the
top level run_kselftest.sh which sources it rather than running it as a
separate script and specifies the shell as /bin/sh. This means that on
systems where /bin/sh is not bash (such as Debian where /bin/sh defaults to
being dash) we see failures:

./run_kselftest.sh: 195: ./kselftest/runner.sh: Syntax error: "(" unexpected (expecting "}")

These bashisms come from this part of the change:

  4. In runner.sh run_one(), get the return value and use ktap helpers for
     all pass/fail reporting. This allows counting pass/fail numbers in the
     main process.

which uses a bash array to track all the subtests being run. Convert this
to use a simple flat variable instead.

Link: https://lore.kernel.org/r/20260416-selftest-fix-readlink-e-v1-2-94e4cabbdec4@kernel.org
Fixes: 2964f6b816c2 ("selftests: Use ktap helpers for runner.sh")
Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>