Fix default alphabetSize of NFARunAutomaton

[zhaih]

Description

The test NFARunAutomaton is using default max codepoint as alphabet size, and the assertion c < alphabetSize failed, which means c is MAX_CODEPOINT

Stacktrace

java.lang.AssertionError
	at __randomizedtesting.SeedInfo.seed([75637BDB16D0FDF0:1030A407D690701]:0)
	at org.apache.lucene.util.automaton.NFARunAutomaton.getCharClass(NFARunAutomaton.java:161)
	at org.apache.lucene.util.automaton.NFARunAutomaton.step(NFARunAutomaton.java:133)
	at org.apache.lucene.util.automaton.NFARunAutomaton.step(NFARunAutomaton.java:98)
	at org.apache.lucene.util.automaton.NFARunAutomaton.run(NFARunAutomaton.java:121)
	at org.apache.lucene.util.automaton.TestNFARunAutomaton.testAcceptedString(TestNFARunAutomaton.java:161)
	at org.apache.lucene.util.automaton.TestNFARunAutomaton.testWithRandomRegex(TestNFARunAutomaton.java:70)
...

NOTE: reproduce with: gradlew test --tests TestNFARunAutomaton.testWithRandomRegex -Dtests.seed=75637BDB16D0FDF0 -Dtests.multiplier=3 -Dtests.slow=true -Dtests.locale=ff-Latn-BF -Dtests.timezone=Europe/Gibraltar -Dtests.asserts=true -Dtests.file.encoding=UTF-8
NOTE: test params are: codec=Asserting(Lucene90): {field=PostingsFormat(name=LuceneVarGapDocFreqInterval)}, docValues:{}, maxPointsInLeafNode=1344, maxMBSortInHeap=6.120921628968405, sim=Asserting(RandomSimilarity(queryNorm=true): {field=org.apache.lucene.search.similarities.BooleanSimilarity@64e5ef99}), locale=ff-Latn-BF, timezone=Europe/Gibraltar
NOTE: Linux 5.11.0-41-generic amd64/Oracle Corporation 16 (64-bit)/cpus=16,threads=1,free=134519392,total=259588096
NOTE: All tests run in this JVM: [TestBKD, TestWeakIdentityMap, TestDaciukMihovAutomatonBuilder, TestCompiledAutomaton, TestNFARunAutomaton]