[REPL] tests: be better about correct IO ordering (#47160)

Particularly relevant for COLUMNS being narrow for some cases, or the
kernel being configured with small IO buffers, or both. Or someone
setting a particularly long path for TMPDIR. Or so on.

Author: vtjnash

stdlib/REPL/test/lineedit.jl

@@ -474,7 +474,8 @@ end
 # julia> is 6 characters + 1 character for space,
 # so the rest of the terminal is 73 characters
 #########################################################################
-let buf = IOBuffer(
+withenv("COLUMNS"=>"80") do
+    buf = IOBuffer(
         "begin\nprint(\"A very very very very very very very very very very very very ve\")\nend")
     seek(buf, 4)
     outbuf = IOBuffer()