testsuite: refactor tests on TAP support in view of future changes

testsuite: refactor tests on TAP support in view of future changes

* tests/defs (fetch_tap_driver): New subroutine; it fetches the
automake-provided TAP driver from the `lib/' directory into the
current directory, and edits its shebang line so that it will be
run with the perl interpreter determined at configure time.
* tests/tap-setup.sh: Use it.
* tests/tap-common-setup.test: There's no need to AC_SUBST `PERL'
anymore, nor to use it in the Makefile to run the TAP driver.
Also, use the `fetch_tap_driver' function instead of copying the
`tap-driver' auxiliary script directly.
* tests/tap-bad-prog.tap: Likewise.
* tests/tap-diagnostic-custom.test: Likewise.
* tests/tap-doc.test: Likewise.
* tests/tap-merge-stdout-stderr.test: Likewise.
* tests/tap-more.test: Likewise.
* tests/tap-more2.test: Likewise.
* tests/tap-recheck.test: Likewise.
* tests/tap-summary-aux.sh: Likewise.
* tests/tap-basic.test: Likewise, and fix a grammaro in comments
since we are at it.