ENHANCEMENT: DSLX: Let formatting bazel rule `xls_dslx_fmt_test` accept lists as input

The `xls_dslx_fmt_test` rule to check if file is correctly formatted requires creating a new rule for each file in the directory. It would be a nice quality of life improvement if `src` could also be `srcs` and function as a list of files to check. Current test rule requires writing repetitively:

```
xls_dslx_fmt_test(
name = "fmt_file_1",
src = "file_1.x",
)

xls_dslx_fmt_test(
name = "fmt_file_2",
src = "file_2.x",
)
...
xls_dslx_fmt_test(
name = "fmt_file_N",
src = "file_N.x",
)
```
Proposed way:
```
xls_dslx_fmt_test(
name = "fmt_all",
srcs = [
"file_1.x",
"file_2.x",
...
"file_N.x",
]
)
```