Fix overflow in Actions Summary - #660

In #589 a regression was introduced. A shortSummary variable (holding a 35+ byte string) was created outside the getReport function and prepended to the step summary, bypassing the Action's truncation logic. For summaries that need truncation, this causes them to still overflow in many cases.

This PR

• adds an optional input to getReport to pass a prependString
• adds prependStringLen as an input to trimReport
• amends the truncation logic of both to account for these extra bytes when present
• adds a test suite to cover the expected truncation behavior for these two functions
  • the length assertions for these test cases are variable based on the length of a single output line rather than exact numbers
  • this is to help avoid them being tied to the byte length of the "Report exceeded GitHub limit" error message, so that it will be easier to change that message in the future

This may resolve issue #654, but needs further confirmation from the reporter.