... before calling erlang:function_exported(ct_testspec,get_tests,1).

Parse given test specs and add all spec- and suite directories as
extra_src_dirs in order to ensure that all these directories are
copied to the _build area and the suites are compiled.

Specs located in the project- or app root are explicitly copied to the
_build area in order to avoid recursive copying of the complete
directory tree.

This is a bugfix. It makes sure that the given path to a testspec is
translated so common_test will pick the spec from the _build
directory, and not from the source tree.

This is necessary in order to automatically get the testspec included
as an artifact (i.e. copied to the _build dir) in the case when it is
stored in another directory than 'test'.

Bug: option 'spec' is not specifically handled when merging options
from the command line with options from rebar.config. Due to this, if
the config specifies a 'spec', then this will take precedence over any
'dir' and/or 'suite' on the command line.

This commit takes special care of all options that can be used to
select tests - meaning that if any of the options 'spec', 'dir',
'suite', 'group' or 'case' are specified on the command line, then all
'spec', 'dir', 'suite', 'group' and 'case' options in rebar.config
will be ignored.

Some tricky changes in there but should be okay

`rebar3 ct --spec foo.spec,bar.spec,baz.spec` now works

also added support for the `join_specs` flag on the command line

- restore path after loading applications,
- helpful comments.

add definition of 'COMMON_TEST' macro to eunit provider

The idea is that given we accept arbitrary config items for CT, we
should similarly be able to pass unsupported options and keep things
running.

However for unsupported options, a warning is very useful to have.

this ensures the project apps are compiled to `lib/` instead of `extras/`

this allows repeated test suite names across apps without conflicts

previously rebar3 dropped suites declared at the root of the project (via
`--suite=whatever_SUITE' probably) and warned. this was because the compiler
would recursively copy and compile everything in the directory indicated by
the test suite. this changes the copy mechanism to only copy erl source files
and directories that end with `_SUITE_data' into the `extras' dir in `_build'

when `ct_opts`, `eunit_tests`, `eunit_first_files`, `ct_first_files`, `erl_first_files`,
`eunit_compile_opts`, `ct_compile_opts` and `erl_opts` have values that are single non-list
terms warn and try wrapping them in a list when processing them in the `eunit` and `ct`
providers

This uses cth_readable to:
- silence error_logger output to the shell unless a test fails
- silence ct:pal output to the shell unless a test fails

I have currently not baked in any way to disable this behaviour, but I
figured if it is required, there is time to do it before the final
3.0.0 release.

* remove partial support for ct `test_spec` until it can be done properly
  and warn if `test_spec` is present in test opts
* use new compiler functionality to reduce complexity of provider
* reduce command line options available to those that can be
  supported properly

at the cost of some SASL warnings this prevents rebar3 from terminating
processes when reloading their code before running tests

* modify compiler interface to work on either application objects or
  directories containing source files
* compile all sources in `src_dirs` to the application `ebin` dir and
  all sources in `extra_src_dirs` to a directory mirroring it's
  position in the app's `_build` directory. for example, `apps/foo/more`
  would compile to `_build/default/lib/foo/more`
  for `extra_src_dirs` in the root of a project with multiple
  applications (so orphan directories that don't "belong" to an
  application) compile to `_build/default/extras/more`
* copy directories specified in `extra_src_dirs` into the `_build`
  directory so tools like `ct` and `xref` that expect source to be
  in a particular location still work
* clean compiled artifacts from all `extra_src_dirs`
* alter `eunit`, `ct` and `cover` to work with the new directory
  structure
* billions of new tests

fixes #788

to itself so a selective receive can be used to retrieve it

fixes #547