[riot-notifications] [RIOT-OS/RIOT] tests/trace: use raw strings in expect regexp (#15370)

[Alexandre Abadie]

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### Contribution description

This PR cleans up the Python automated test script of the `tests/trace` application: regexp string should be marked as raw string to avoid the following flake8 errors:

```
tests/sys/trace/tests/01-run.py:8:28: W605 invalid escape sequence '\ '
tests/sys/trace/tests/01-run.py:8:31: W605 invalid escape sequence '\d'
tests/sys/trace/tests/01-run.py:9:28: W605 invalid escape sequence '\+'
tests/sys/trace/tests/01-run.py:9:30: W605 invalid escape sequence '\ '
tests/sys/trace/tests/01-run.py:9:33: W605 invalid escape sequence '\d'
```

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### Testing procedure

- A green CI
- `make -C tests/trace all test` is successful:

<details>

```
$ make -C tests/trace all test --no-print-directory 
Building application "tests_trace" for "native" with MCU "native".

"make" -C /work/riot/RIOT/boards/native
"make" -C /work/riot/RIOT/boards/native/drivers
"make" -C /work/riot/RIOT/core
"make" -C /work/riot/RIOT/cpu/native
"make" -C /work/riot/RIOT/cpu/native/periph
"make" -C /work/riot/RIOT/cpu/native/stdio_native
"make" -C /work/riot/RIOT/drivers
"make" -C /work/riot/RIOT/drivers/periph_common
"make" -C /work/riot/RIOT/sys
"make" -C /work/riot/RIOT/sys/auto_init
"make" -C /work/riot/RIOT/sys/div
"make" -C /work/riot/RIOT/sys/test_utils/interactive_sync
"make" -C /work/riot/RIOT/sys/trace
"make" -C /work/riot/RIOT/sys/xtimer
/usr/bin/ld: /work/riot/RIOT/tests/trace/bin/native/cpu/tramp.o: warning: relocation against `_native_saved_eip' in read-only section `.text'
/usr/bin/ld: warning: creating DT_TEXTREL in a PIE
   text	   data	    bss	    dec	    hex	filename
  32670	    596	  48380	  81646	  13eee	/work/riot/RIOT/tests/trace/bin/native/tests_trace.elf
r
/work/riot/RIOT/tests/trace/bin/native/tests_trace.elf  
RIOT native interrupts/signals initialized.
LED_RED_OFF
LED_GREEN_ON
RIOT native board initialized.
RIOT native hardware initialization complete.

Help: Press s to start test, r to print it is ready
READY
s
START
main(): This is RIOT! (Version: 2021.01-devel-568-gef871-pr/tests/trace_script_cleanup)
n=   0 t=  3056935 v=0x00000000
n=   1 t=+       3 v=0x00000001

```

</details>

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### Issues/PRs references

Raised when running the static test locally in #15358

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You can view, comment on, or merge this pull request online at:

  https://github.com/RIOT-OS/RIOT/pull/15370

-- Commit Summary --

  * tests/trace: declare expect regexp as raw strings

-- File Changes --

    M tests/trace/tests/01-run.py (4)

-- Patch Links --

https://github.com/RIOT-OS/RIOT/pull/15370.patch
https://github.com/RIOT-OS/RIOT/pull/15370.diff

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