Added tests for ToString throwing in String.prototype methods

[G-Lee1031]

Hello.
This PR adds tests covering two cases where ToString returns an abrupt completion.
The changes are applied to String.prototype.replace, String.prototype.slice, String.prototype.substring, String.prototype.toLowerCase.

In all sections, the following two abrupt completion cases are tested:

• When the argument is a Symbol (throws TypeError).
• When ToPrimitive returns an abrupt completion (specifically, by passing { valueOf: undefined, toString: undefined } to trigger a throw in OrdinaryToPrimitive step 4)

String.prototype.replace

• tostring-this-throws-symbol.js: tests Symbol case
• tostring-this-throws-toprimitive.js: tests the case where ToPrimitive returns an abrupt completion

String.prototype.slice

• this-value-tostring-throws-symbol.js: tests Symbol case
• this-value-tostring-throws-toprimitive.js: tests the case where ToPrimitive returns an abrupt completion

String.prototype.substring

• this-value-tostring-throws-symbol.js: tests Symbol case
• this-value-tostring-throws-toprimitive.js: tests the case where ToPrimitive returns an abrupt completion

String.prototype.toLowerCase

• this-value-tostring-throws-symbol.js: tests Symbol case
• this-value-tostring-throws-toprimitive.js: tests the case where ToPrimitive returns an abrupt completion

I'm sorry if the duplicate filenames cause any confusion. My intention was to keep the structure consistent.

[ptomato]

Thanks!