twister: Don't run coverage report when --build-only is used

When doing a split build and test with coverage enabled, (i.e.
`twister --coverage --build-only` followed by `twister --coverage
--test-only`), Twister attempts to run coverage reports immediately
after building, which fails because tests haven't actually run yet
and causes several error messages to print in to the log.

This change causes twister to skip the call to `run_coverage()` if
`--build-only` is set and print an info message. This suppresses the
error messages from the coverage report tool complaining about missing
coverage files, but still instruments the built code for coverage data
collection. When twister is invoked again, but with `--test-only`
instead, the coverage files will be emitted and the reports
successfully generated.

 #### Testing

I ran...

```
$ zephyr/scripts/twister -T zephyr/tests/lib/cmsis_dsp/basicmath/
--coverage -p native_posix --build-only
$ zephyr/scripts/twister -T zephyr/tests/lib/cmsis_dsp/basicmath/
--coverage -p native_posix --test-only
```

... followed by a combined build and test ...

```
$ zephyr/scripts/twister -T zephyr/tests/lib/cmsis_dsp/basicmath/
--coverage -p native_posix
```

... as a control and diffed the HTML coverage reports, which were
identical other than a timestamp.

Signed-off-by: Tristan Honscheid <honscheid@google.com>