BZ1099757: vars.exp: fix and strengthen

BZ1099757: vars.exp: fix and strengthen

Since PR16615 (merge commit 4636ca3), linenos in statement probes have
become more precise and stringent. As a result, vars.exp was failing
because it was trying to probe a statement at a lineno with no line
records.

We fix this by first retrieving the first valid lineno using stap -l.
The test has also been refactored in general to be more robust. We can
now more easily add more functions to test (e.g. we now test both
bio_copy_user() and vfs_read()).