VF.reset() to restore all globals to their default values.

[ronyeh]

• Add a VF.reset() that restores all globals to it's original state. (Ideally, we'll have a class to capture all the globals to simplify this.
• Update our test harness (VexFlowTests.runTest(...)) code to call VF.reset() before each test begins.

This was discussed in https://github.com/0xfe/vexflow/issues/1109#issuecomment-902608274