Skip blanks before parsing simple command word

We always have to skip blanks before parsing a simple command word, so
it should be natural to skip them in a simple command word parser.

I chose not to skip blanks after the word because that would make the
parser algorithm more complex and require another blank skipper to be
applied before the simple command word parser.
Move AssignmentParserStub to AssignmentParserTestHelper.hh

AssignmentParserStub is going to be used by another test.
Move SimpleCommandWordParserStub to SimpleCommandWordParserTestHelper.hh

SimpleCommandWordParserStub is going to be used by another test.