In implementing this test one of the assertions assumes that the
priority is the default in linux (0) but, evidently, some of the build
runners prioritize the test to, at least, 5. This ensures that
regardless of the priority the test passes by validating that its the
process's priority
```
fgrim@host001 :~/llvm-project/debug_build> nice -n 15 tools/lldb/unittests/Process/elf-core/ProcessElfCoreTests
[==========] Running 2 tests from 1 test suite.
[----------] Global test environment set-up.
[----------] 2 tests from ElfCoreTest
[ RUN ] ElfCoreTest.PopulatePrpsInfoTest
[ OK ] ElfCoreTest.PopulatePrpsInfoTest (4 ms)
[ RUN ] ElfCoreTest.PopulatePrStatusTest
[ OK ] ElfCoreTest.PopulatePrStatusTest (3 ms)
[----------] 2 tests from ElfCoreTest (7 ms total)
[----------] Global test environment tear-down
[==========] 2 tests from 1 test suite ran. (8 ms total)
[ PASSED ] 2 tests.
===(10:03)===
fgrim@host001 :~/llvm-project/debug_build>
```
To create elf core files there are multiple notes in the core file that
contain these structs as the note. These populate methods take a Process
and produce fully specified structures that can be used to fill these
note sections. The pr also adds tests to ensure these structs are
correctly populated.
