fix(FasterXML#291): InstantDeserializer fail parse neg num

Full Issue Title: InstantDeserializer fails to parse negative numeric
timestamp strings for pre-1970 values.

- Updated `InstantDeserializer._countPeriods(String)` to check for `-`
  and `+` at beginning of numeric string.
- Additionally, added check if `ALLOW_LEADING_PLUS_SIGN_FOR_NUMBERS` is
  enabled. If disabled, throws same exception as before change from
  `_countPeriods(...)`.
- Moved associated tests out of `failing` directory

datetime/src/main/java/com/fasterxml/jackson/datatype/jsr310/deser/InstantDeserializer.java

@@ -44,6 +44,8 @@
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
 
+import static com.fasterxml.jackson.core.json.JsonReadFeature.ALLOW_LEADING_PLUS_SIGN_FOR_NUMBERS;
+
 /**
  * Deserializer for Java 8 temporal {@link Instant}s, {@link OffsetDateTime},
  * and {@link ZonedDateTime}s.
@@ -380,11 +382,16 @@ protected boolean shouldReadTimestampsAsNanoseconds(DeserializationContext conte
     }
 
     // Helper method to find Strings of form "all digits" and "digits-comma-digits"
-    protected int _countPeriods(String str)
+    protected int _countPeriods(JsonParser p, String str)
     {
         int commas = 0;
-        for (int i = 0, end = str.length(); i < end; ++i) {
-            int ch = str.charAt(i);
+        int i = 0;
+        int ch = str.charAt(i);
+        if (ch == '-' || (ch == '+' && p.isEnabled(ALLOW_LEADING_PLUS_SIGN_FOR_NUMBERS.mappedFeature()))) {
+            ++i;
+        }
+        for (int end = str.length(); i < end; ++i) {
+            ch = str.charAt(i);
             if (ch < '0' || ch > '9') {
                 if (ch == '.') {
                     ++commas;
@@ -413,7 +420,7 @@ protected T _fromString(JsonParser p, DeserializationContext ctxt,
                 _formatter == DateTimeFormatter.ISO_ZONED_DATE_TIME
             ) {
             // 22-Jan-2016, [datatype-jsr310#16]: Allow quoted numbers too
-            int dots = _countPeriods(string);
+            int dots = _countPeriods(p, string);
             if (dots >= 0) { // negative if not simple number
                 try {
                     if (dots == 0) {

datetime/src/test/java/com/fasterxml/jackson/datatype/jsr310/failing/InstantDeserializerNegativeNumericTimestampString291Testest.java → datetime/src/test/java/com/fasterxml/jackson/datatype/jsr310/deser/InstantDeser291Test.java

@@ -1,4 +1,4 @@
-package com.fasterxml.jackson.datatype.jsr310.failing;
+package com.fasterxml.jackson.datatype.jsr310.deser;
 
 import java.time.Instant;
 import java.util.Locale;
@@ -18,7 +18,7 @@
 
 // [modules-java8#291] InstantDeserializer fails to parse negative numeric timestamp strings for
 //   pre-1970 values.
-public class InstantDeserializerNegativeNumericTimestampString291Testest 
+public class InstantDeser291Test 
     extends ModuleTestBase
 {
     private final JsonMapper MAPPER = JsonMapper.builder()