Bump reference platform to Ubuntu 18.04 LTS (#3037)

Ubuntu 14.04 reached its end of life on April 30m 2019.
Let's bump the reference to the latest LTS, which is 18.04.

Ubuntu 18.04 has newer Pylint and Cppcheck, the necessary
fixes and suppresses are also included in this PR.

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

tests/unit-core/test-arraybuffer.c

@@ -304,6 +304,7 @@ main (void)
   /* Test ArrayBuffer external memory map/unmap */
   {
     const uint32_t buffer_size = 20;
+    /* cppcheck-suppress variableScope */
     JERRY_VLA (uint8_t, buffer_p, buffer_size);
     {
       jerry_value_t input_buffer = jerry_create_arraybuffer_external (buffer_size, buffer_p, NULL);

tests/unit-core/test-typedarray.c

@@ -155,7 +155,7 @@ test_typedarray_queries (test_entry_t test_entries[]) /**< test cases */
  */
 static
 void test_buffer_value (uint64_t value, /**< value to test for */
-                        const uint8_t *buffer, /**< buffer to read value from */
+                        const void *buffer, /**< buffer to read value from */
                         uint32_t start_offset, /**< start offset of the value */
                         jerry_typedarray_type_t typedarray_type, /**< type of TypedArray */
                         uint32_t bytes_per_element) /**< bytes per element for the given type */
@@ -518,11 +518,11 @@ main (void)
   /* Test TypedArray operations in js */
   {
     const uint32_t element_count = 14;
-    uint8_t expected_value = 42;
 
     jerry_value_t array = jerry_create_typedarray (JERRY_TYPEDARRAY_UINT8, element_count);
 
     {
+      uint8_t expected_value = 42;
       JERRY_VLA (uint8_t, expected_data, element_count);
       memset (expected_data, expected_value, element_count);