tests: avoid extra forks in the testsuite

tests: avoid extra forks in the testsuite

* tests/shell-or-perl: Prefer the `read' builtin over `grep' to
look at the shebang line of test scripts.  Since `read' is a
special builtin, it might abort the whole program upon failures,
so add extra sanity checks, verifying that the test script exists
and is readable, before trying to read from it.