RFR: 8257147: [TESTBUG] Set a larger default loop count for the VectorAPI jtreg tests

[Xiaohong Gong]

The current default loop count of VectorAPI tests is too small (10 for load/store tests, 100 for arithmetic tests) to make the tests compiled with C2. This makes some potential issues not be reported as expected.

This patch fixes it by setting a larger default iteration.

The whole tests running time increases with this patch. Here is the results on two different types of machines:

Running with "`-conc:10`":
             Before     After
System A     5m30s      6m43s
System B     5m1s       6m31s

Running with "`-conc:1`":
             Before     After
System A     26m52s     45m6s
System B     30m3s      43m30s

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Commit messages:
 - 8257147: [TESTBUG] Set a larger default loop count for the VectorAPI jtreg tests

Changes: https://git.openjdk.java.net/jdk/pull/1621/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1621&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8257147
  Stats: 94 lines in 63 files changed: 0 ins; 0 del; 94 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1621.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1621/head:pull/1621

PR: https://git.openjdk.java.net/jdk/pull/1621