clar: add tap output option

Add an output abstraction layer, with a single output format, "clap",
the clar protocol, which is the current output format for clar.

We want to parse arguments before we start printing any output; the
arguments themselves may impact the way we display that output.

When using `-s` to specify a particular test, it will do a prefix match.
Thus, `-sapply::both::rename_a_to_b_to_c` will match both a test named
`test_apply_both__rename_a_to_b_to_c` and a test that begins with that
name, like `test_apply_both__rename_a_to_b_to_c_exact`.

Permit a trailing `$` to `-s` syntax.  This allows a user to specify
`-sapply::both::rename_a_to_b_to_c$` to match _only_ the
`test_apply_both__rename_a_to_b_to_c` function.

We already filter to ensure that the given prefix matches the current
test name.  Also ensure that the length of the test name matches the
length of the filter, sans trailing `$`.

As different test suites for our CI are mostly defined via CMake, it's
hard to run those tests with a summary file path as that'd require us to
add another parameter to all unit tests. As we do not want to
unconditionally run unit tests with a summary file, we would have to add
another CMake build parameter for test execution, which is ugly.

Instead, implement a way to provide a summary file path via the
environment.

We use the `__LINE__` macro in several places throughout clar to allow
easier traceability when e.g. a test fails. While `__LINE__` is of type
`size_t`, the clar functions all accept an integer and thus may loose
precision. While unlikely that any file in our codebase will exceed a
linecount of `INT_MAX`, let's convert it anyway to silence any compiler
warnings.

Instead of trying to have a clever iterator pattern that increments the
error number, just iterate over errors in the report errors or report
all functions as it's easier to reason about in this fashion.