Test config rename

Update automated nightly testing to use new logfile names and email content
based on human readable configuration tags (as opposed to the unwieldly system
previously used). The new system requires an environment variable,
`CHPL_NIGHTLY_TEST_CONFIG_NAME`, which is used for both the log filename and in
the email subject line. All tests scripts set this variable before calling
nightly.

This value of this variable is unrestricted, at least from
`util/cron/nightly`'s perspective. The team has agreed upon a simple naming
scheme that relies on good judgement and simplicity.

In addition, the nightly emails now include a "Tests run:" section in the
header, which highlights the test directories that were passed to
start_test. This will be empty when the "full suite" of tests is run. The
output of `printchplenv --debug` is also appended to the bottom of the email.

In order to support "Tests run:" in general, a new environment variable,
`CHPL_NIGHTLY_TEST_DIRS`, is supported by nightly. When set, the directories in
that variable will be added to the start_test call. It works with or without
other flags, like `-examples` or `-multilocale`. See the LLVM test scripts for
an example usage.