test: exclude gperf files from captured coverage

test: exclude gperf files from captured coverage

Recent lcov started complaining loudly about unknown lines in gperf
files:

...
Found gcov version: 13.2.1
Using intermediate gcov format
Recording 'internal' directories:
...
Finished processing 1634 GCNO files
Apply filtering..
Message summary:
  1 error message:
    range: 1
  28 warning messages:
    gcov: 27
    usage: 1
geninfo: ERROR: (range) unknown line '33' in /build/src/home/homed-gperf.gperf: there are only 22 lines in the file.
  Use 'geninfo --filter range' to remove out-of-range lines.
        (use "geninfo --ignore-errors range ..." to bypass this error)

Since we drop the coverage of built files from the final report anyway,
let's do it also when capturing both initial and real coverage to avoid
this error.