BaseTools/Plugin/HostBaseUnitTestRunner: Enable gtest xml output

BaseTools/Plugin/HostBaseUnitTestRunner: Enable gtest xml output

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=4134

Set environment variable GTEST_OUTPUT to specify the output
format of XML and the output file name.  Both CMOCKA_XML_FILE
and GTEST_OUTPUT are set for each host based unit test to
support both cmocka unit tests and gtest unit tests.

Cc: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Yuwei Chen <yuwei.chen@intel.com>
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Bob Feng <bob.c.feng@intel.com>