Implement SRFI-64 - A Scheme API for test suites.

Implement SRFI-64 - A Scheme API for test suites.

* module/srfi/srfi-64.scm: New file.
* module/srfi/srfi-64/testing.scm: New file.
* module/Makefile.am: Add rule for srfi-64.go dependency on
  srfi-64/testing.scm.
  (SRFI_SOURCES): Add srfi/srfi-64.scm.
  (NOCOMP_SOURCES): Add srfi/srfi-64/testing.scm.
* doc/ref/srfi-modules.texi (SRFI-64): New node.
* test-suite/tests/srfi-64.test: New file.
* test-suite/tests/srfi-64-test.scm: New file.
* test-suite/Makefile.am (SCM_TESTS): Add test-suite/tests/srfi-64.test.
  (EXTRA_DIST): Add tests/srfi-64-test.scm.