documentclass[12pt]{article}
usepackage[utf8]{inputenc}
usepackage[T1]{fontenc}
begin{document} noindent
Test emph{test} test.newline % This is working
Test~emph{test}~test.newline % Extra space before and after emph{}
Testemph{test}test.newline % Extra space before and after emph{}

noindent
Test.footnote{Test emph{test} test.}newline % Missing space after emph{}
Test.footnote{Testemph{test} test.}newline %Extra space before and missing space after emph{}
Test.footnote{Test emph{test}test.} %Missing space before and extra space after emph{}
end{document} ``