When tests are compiled, sourceMappingUrl is not added to the resulting _bin.js file

When I invoke closure_js_test like so:

```python
closure_js_test(
name = "sometest",
entry_points = ...,
srcs = ...,
deps = ...,
language = "ECMASCRIPT6_STRICT",
defs = [
"--jscomp_error=*",
"--jscomp_off=analyzerChecks",
"--dependency_mode=LOOSE"
],
)
```

the resulting _bin.js file doesn't contain

```javascript
//# sourceMappingURL=sometest_bin.js.map
```

which would allow browsers to use the source map.

Start at the closure_js_test rule implementation and trace how the resulting _bin.js file and its source map are produced. Reproduce the issue with the shown rule configuration, then verify that the generated file includes the sourceMappingURL comment pointing to sometest_bin.js.map.

Written by the indexing model from the issue text.