Allow user to specify profile sort column on the command line. (#4056)

- Add -P <STAT> argument so that caller can specify a sort column for
  cProfile. Can specify multiple columns with commas. e.g.:
      spack -P cumtime,module

- Add --lines option to Spack spec to control number of profile lines
  displayed

- Sort by time by default (because it works in all Python versions)

- Show sort column options in command help.

- Do a short profile run in the unit tests.

share/spack/qa/run-unit-tests

@@ -20,6 +20,9 @@ cd "$SPACK_ROOT"
 # Print compiler information
 spack config get compilers
 
+# Profile and print top 20 lines for a simple call to spack spec
+${coverage_run} bin/spack -p --lines 20 spec mpileaks
+
 # Run unit tests with code coverage
 ${coverage_run} bin/spack test "$@"
 ${coverage_combine}