UnitTestFrameworkPkg/UnitTestLib: Fix target mode log messages

UnitTestFrameworkPkg/UnitTestLib: Fix target mode log messages

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2806

Update the log messages generated in target mode to use
FileName instead of FunctionName.  FunctionName is an empty
string so the log messages generated do not provide enough
information to know the source of a unit test failure.

Using FileName combined with LineNumber provides the
right information to identify the location of a unit test
failure.

Cc: Sean Brogan <sean.brogan@microsoft.com>
Cc: Bret Barkelew <Bret.Barkelew@microsoft.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Bret Barkelew <Bret.Barkelew@microsoft.com>