* shell.m4: Rename as...

* shell.m4: Rename as...
* m4sh.m4: this.
* tests/m4sugar.m4, tests/shell.m4, tests/torture.m4: Rename as...
* tests/m4sugar.at, tests/m4sh.at, tests/torture.at: these.
* tests/semantics.m4, tests/base.m4, tests/suite.m4: Rename as...
* tests/semantics.at, tests/base.at, tests/suite.at: these.
* tests/tools.m4, tests/update.m4, tests/syntax.m4: Rename as...
* tests/tools.at, tests/update.at, tests/syntax.at: these.
* tests/mktests.sh: Adjust the output file names.