Re-enable XML tests in flag_parser_test.cpp

Introduces a dependency on libxml2.

Restores parts of tests deleted in #853

Bug: b/389804379
Test: bazel test '//...'

Author: Databean

base/cvd/MODULE.bazel

@@ -11,6 +11,7 @@ bazel_dep(name = "fmt", version = "10.2.1")
 bazel_dep(name = "gflags", version = "2.2.2")
 bazel_dep(name = "googletest", version = "1.15.2")
 bazel_dep(name = "jsoncpp", version = "1.9.5")
+bazel_dep(name = "libxml2", version = "2.13.5")
 bazel_dep(name = "protobuf", version = "29.1")
 bazel_dep(name = "tinyxml2", version = "10.0.0")
 bazel_dep(name = "zlib", version = "1.3.1.bcr.3")