Make Date.parse() ECMA-262 conform (#3314)

Changes:
- Parse output of Date.prototype.toString() and Date.prototype.toUTCString()
- Date.prototype.toString() is ECMA-262 v9 conform now, only TZ part changed
  (Before ECMA-262 v9 it was implementation-dependent.)
- Reused day_names_p and month_names_p arrays (and made them more efficient)
- Tests updated and new tests added

Fixes #2946.

JerryScript-DCO-1.0-Signed-off-by: Csaba Osztrogonác oszi@inf.u-szeged.hu

jerry-core/ecma/builtin-objects/ecma-builtin-helpers.h

@@ -146,6 +146,9 @@ typedef enum
 } ecma_date_timezone_t;
 
 /* ecma-builtin-helpers-date.c */
+extern const char day_names_p[7][3];
+extern const char month_names_p[12][3];
+
 ecma_number_t ecma_date_day (ecma_number_t time);
 ecma_number_t ecma_date_time_within_day (ecma_number_t time);
 ecma_number_t ecma_date_year_from_time (ecma_number_t time);