Add some tests to gcstress knownfailures

Several tests were broken under gcstress by the new libbacktrace based
gcstress.

The tests are:
  * 151-OpenFileLimit, the new gcstress needs to be able to open
    /proc/self/maps to function.
  * 530-checker-lse2, The test times out due to the 30% slowdown.
  * 712-varhandle-invocations & 624-checker-stringops, The new
    gcstress with its more accurate thread walking seems to expose
    gc-bugs through these tests. See b/111545159.

Bug: 111544552
Bug: 111543628
Bug: 111545159
Test: ./test/testrunner/testrunner.py --host --gcstress
Change-Id: I08d93e2c794c2b91aa5c0ed1c503846f17657231