[clangd][test] Fix -Wmissing-field-initializers in DiagnosticsTests.cpp (NFC)

llvm-project/clang-tools-extra/clangd/unittests/DiagnosticsTests.cpp:921:45:
error: missing field 'Annotations' initializer [-Werror,-Wmissing-field-initializers]
       TextEdit{Main.range("virtual1"), ""}}};
                                            ^
llvm-project/clang-tools-extra/clangd/unittests/DiagnosticsTests.cpp:926:45:
error: missing field 'Annotations' initializer [-Werror,-Wmissing-field-initializers]
       TextEdit{Main.range("virtual2"), ""}}};
                                            ^
2 errors generated.