Make extraction tests work without ${USER} (fixes #378)

[hartwork]

Before

# USER= make -C tests/lcov/extract/ check 
make: Entering directory '[..]/tests/lcov/extract'
Starting tests
lcov/extract/extract.sh ........... [fail] (time 4.7s, mem 58.8MB)
    Skipping 228 previous lines (see [..]/tests//test.log)
    ...
      functions...: 28.6% (2 of 7 functions)
      branches....: 33.3% (8 of 24 branches)
    Filter suppressions:
      region:
        2 instances
        5 coverpoints
    Message summary:
      no messages were reported
    Error:  did not find expected context field
  EXITCODE ...: 1
1 tests executed, 0 passed, 1 failed, 0 skipped (time 4.7s, mem 58.8MB)
Result log stored in [..]/tests/test.log
make: *** [../../common.mak:116: check] Error 1
make: Leaving directory '[..]/tests/lcov/extract'

After

# USER= make -C tests/lcov/extract/ check 
make: Entering directory '[..]/tests/lcov/extract'
Starting tests
lcov/extract/extract.sh ........... [pass] (time 23.1s, mem 58.9MB)
1 tests executed, 1 passed, 0 failed, 0 skipped (time 23.1s, mem 58.9MB)
Result log stored in [..]/tests/test.log
make: Leaving directory '[..]/tests/lcov/extract'