defmt-test: attribute test progress message to the test in question

[jonas-schievink]

Sets the span of the tokens making up the defmt::info! call to the test function's identifier.

Fixes #425

 INFO  (1/11) running `split_filters`...
└─ integration::tests::__defmt_test_entry @ tests/integration.rs:18
 INFO  (2/11) running `basic_roundtrip`...
└─ integration::tests::__defmt_test_entry @ tests/integration.rs:43
 INFO  (3/11) running `no_filters_no_frames`...
└─ integration::tests::__defmt_test_entry @ tests/integration.rs:58
 INFO  (4/11) running `filter_mask32_std`...
└─ integration::tests::__defmt_test_entry @ tests/integration.rs:68
 INFO  (5/11) running `filter_mask32_ext`...
└─ integration::tests::__defmt_test_entry @ tests/integration.rs:112
 INFO  (6/11) running `filter_mask16`...
└─ integration::tests::__defmt_test_entry @ tests/integration.rs:153
 INFO  (7/11) running `filter_list32_std`...
└─ integration::tests::__defmt_test_entry @ tests/integration.rs:193
 INFO  (8/11) running `filter_list32_ext`...
└─ integration::tests::__defmt_test_entry @ tests/integration.rs:226
 INFO  (9/11) running `dequeue_lower_priority_frame`...
└─ integration::tests::__defmt_test_entry @ tests/integration.rs:264
 INFO  (10/11) running `enable_non_blocking`...
└─ integration::tests::__defmt_test_entry @ tests/integration.rs:317
 INFO  (11/11) running `ext_roundtrip`...
└─ integration::tests::__defmt_test_entry @ tests/integration.rs:330

(this was pretty painful to test, since qemu-run does not print location information, and probe-run doesn't use git defmt out of the box, so I had to edit its Cargo.toml, and then make bxcan use git defmt)

[japaric]

Nice! Looks good to me. I would appreciate if this waits until PR #531 is merged though

[japaric]

I would appreciate if this waits until PR #531 is merged though

no, wait. this is the other macros crate. clear to land then

bors r+

[bors]

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