End-to-end tests

[jsanford8]

Add a full set of end-to-end tests for the main helm editor. Examples of things that need to be tested:

• clicking load displays the modal dialog
• clicking the x hides the modal dialog
• clicking outside of the modal dialog hides the modal dialog
• loading in an incorrect PEPTIDE sequence results in an error message
• loading in an incorrect NUCLEOTIDE (RNA/DNA) sequence results in an error message
• loading in a correct/known PEPTIDE sequence results in valid HELM and valid SVG display
• loading in a correct/known NUCLEOTIDE sequence results in valid HELM and valid SVG display
• loading in a known-incorrect HELM sequence results in an error message
• loading in a known-correct HELM sequence results in the HELM string and valid SVG display
• - test a single polymer
• - test a HELM sequence with multiple polymers
• - test a HELM sequence with a single cycle
• - test a HELM sequence with two cycles - should result in warning
• toggling the lower pane flips from HELM to Canonical HELM to Image, etc

(there are likely more, but that would be a good start)