docs, tests: synchronize examples from docs to tests

docs, tests: synchronize examples from docs to tests

* tests/README (Writing test cases): Give suggestions on how to
keep test cases and examples in the documentation synchronized.
* doc/automake.texi: Improve or fix existing testcase-referencing
comments, and add many new ones.
* HACKING (Administrivia): Suggest to test complex examples and
idioms from the manual.
* tests/specflg8.test: Improve synchronization with the example
in the manual.
* tests/output11.test:Likewise.
* tests/txinfo21.test:Likewise.
* tests/interp.test: Likewise.  Since we are at it, and enable
the `errexit' shell flag, do related changes, and add trailing
`:'command.
* tests/amhello-cflags.test: New test.
* tests/amhello-cross-compile.test: Likewise.
* tests/amhello-binpkg.test: Likewise.
* tests/tests-environment-backcompat: Likewise.
* tests/parallel-tests-log-compiler-example.test: Likewise.
* tests/Makefile.am (TESTS): Update.