tikz parallel foreach

You define parallel foreach as being a style with parameter text #1in#2via#3, with no spaces, and then you use:

parallel foreach = x in {test} via c

so the arguments to parallel foreach are ␣x␣, ␣{test}␣, and ␣c, then when you pass it to getitem, you have:

clist_gset:Nng_list_clist␣{test}␣

The first space token is consumed by TeX when grabbing the second argument to clist_gset:Nn, but the second space is typeset in your node.

There are several possibilities here, for instance tl_trim_spaces:n:

clist_gset:Nx g_list_clist { tl_trim_spaces:n {#1} }

(actually, the implementation is a bit more complicated to check for braces around the argument to parallel foreach; see the code at the bottom of this answer). However expl3 provides clist_item:nn, so you can take items from a comma-separated list without storing it in a variable, which makes things simpler here:

exp_args:Nx clist_item:nn
  { tl_trim_spaces:n {#1} } {#2}

Though this doesn't cope with the possibility of multiple items in the list because of the extra braces. You need to check if the argument to parallel foreach is within braces, and if so remove them, with something like:

NewDocumentCommand {getitem} {m O{1}} {
  exp_args:Nx clist_item:nn
    { tl_trim_spaces_apply:nN {#1} zhiyuan_remove_braces:n } {#2}
}
cs_new:Npn zhiyuan_remove_braces:n #1
  {
    tl_if_head_is_group:nTF {#1}
      { exp_not:n #1 }
      { exp_not:n {#1} }
  }

With that, your code produces the expected output:

documentclass[tikz, border=1cm]{standalone}
usepackage{xparse}
ExplSyntaxOn
NewDocumentCommand {getitem} {m O{1}} {
  exp_args:Nx clist_item:nn
    { tl_trim_spaces_apply:nN {#1} zhiyuan_remove_braces:n } {#2}
}
cs_new:Npn zhiyuan_remove_braces:n #1
  {
    tl_if_head_is_group:nTF {#1}
      { exp_not:n #1 }
      { exp_not:n {#1} }
  }
ExplSyntaxOff
pgfset{
  foreach/parallel foreach/.style args={#1in#2via#3}{
    evaluate=#3 as #1 using "beforegetitem{#2}[#3]"
  },
}

begin{document}
begin{tikzpicture}
  foreach i [
    count = c,
    parallel foreach = x in {test,another} via c,
  ] in {1,2}
    node at (1, i) {blankx};
    node at (1, -1) {no blankgetitem{{test}}[1]};
end{tikzpicture}
end{document}

And here's the code using a clist variable rather than clist_item:nn:

documentclass[tikz, border=1cm]{standalone}
usepackage{xparse}
ExplSyntaxOn
clist_new:N g_list_clist
NewDocumentCommand {getitem} {m O{1}} {
  use:x
    {
      clist_gset:Nx exp_not:N g_list_clist
        { tl_trim_spaces_apply:nN {#1} zhiyuan_remove_braces:n }
    }
  clist_item:Nn g_list_clist {#2}
}
cs_new:Npn zhiyuan_remove_braces:n #1
  {
    tl_if_head_is_group:nTF {#1}
      { exp_not:n #1 }
      { exp_not:n {#1} }
  }
ExplSyntaxOff
pgfset{
  foreach/parallel foreach/.style args={#1in#2via#3}{
    evaluate=#3 as #1 using "beforegetitem{#2}[#3]"
  },
}

begin{document}
begin{tikzpicture}
  foreach i [
    count = c,
    parallel foreach = x in {test,another} via c,
  ] in {1,2}
    node at (1, i) {blankx};
    node at (1, -1) {no blankgetitem{{test}}[1]};
end{tikzpicture}
end{document}