Fix spawn test when env BAR is set

[omus]

When running the "spawn" tests I noticed this failure:

spawn                              (7) |         failed at 2021-01-12T14:23:55.599
Test Failed at /Users/omus/Development/Julia/x86/latest-src/usr/share/julia/test/spawn.jl:718
  Expression: strip(String(read(cmd))) == "foo"
   Evaluated: "foo 2" == "foo"

I discovered that the issue was that I had set BAR=2 in my shell environment before starting Julia and running the tests. The change here ensures that the test doesn't rely on the user not defining BAR.

One thing to note. This issue only seems to appear when you have compiled the Julia dependencies from source and doesn't occur when using BinaryBuilder dependencies.

[omus]

Reproducing the failure can be done with:

export BAR=2
julia -e '
    using Test
    shcmd = "sh"
    withenv("FOO" => "foo") do
        cmd = Cmd(`$shcmd -c "echo \$FOO \$BAR"`)
        @test strip(String(read(cmd))) == "foo"
    end'