Testsuite: Use `$PATH_SEPARATOR', not `:', when extending PATH.

Testsuite: Use `$PATH_SEPARATOR', not `:', when extending PATH.

* tests/compile2.test: Do no uselessly (implicitly) repeat the
computation of PATH_SEPARATOR again.
* tests/instmany-mans.test: Use `$PATH_SEPARATOR', not `:', when
extending/redefining PATH.
* tests/instmany-python.test: Likewise.
* tests/instmany.test: Likewise.
* tests/man4.test: Likewise.
* tests/mkinst3.test: Likewise.
* tests/mmodely.test: Likewise.
* tests/multlib.test: Likewise.
* tests/txinfo30.test: Likewise.
* tests/README (Section "Writing test cases" subsection "Do"):
Updated.
* Makefile.am (sc_tests_PATH_SEPARATOR): New maintainer check.
(syntax_check_rules): Updated.