tests/functional/repl: Improve precision and readability

... as well as match buildReadlineNoMarkdown.

Unfortunately it doesn't support long inputs or multiline inputs
for now.
This needs to make better use of the interacter interface.

tests/functional/repl.sh

@@ -262,6 +262,14 @@ badExitCode=0
 
 nixVersion="$(nix eval --impure --raw --expr 'builtins.nixVersion' --extra-experimental-features nix-command)"
 
+# I couldn't get readline and editline to agree on the newline before the prompt,
+# so let's just force it to be one empty line. Ideally we get the two to agree
+# or use a simpler interacter for testing.
+stripEmptyLinesBeforePrompt() {
+  # --null-data:  treat input as NUL-terminated instead of newline-terminated
+  sed --null-data 's/\n\n*nix-repl>/\n\nnix-repl>/g'
+}
+
 runRepl () {
 
   # That is right, we are also filtering out the testdir _without underscores_.
@@ -273,8 +281,11 @@ runRepl () {
   testDirNoUnderscores="${testDir//_/}"
 
   # TODO: pass arguments to nix repl; see lang.sh
+  _NIX_TEST_RAW_MARKDOWN=1 \
+  _NIX_TEST_REPL_ECHO=1 \
   nix repl 2>&1 \
     | stripColors \
+    | stripEmptyLinesBeforePrompt \
     | sed \
       -e "s@$testDir@/path/to/tests/functional@g" \
       -e "s@$testDirNoUnderscores@/path/to/tests/functional@g" \