Implement --force-stdin-eval-mode command line arg

- Takes 'interactive' and 'non-interactive' as valid values
- Provides the means to switch between REPL and plain eval STDIN
    without depending on whether STDIN is a TTY or not. At the
    very least, this lets us easily write REPL tests.

src/HLL/Compiler.nqp

@@ -23,7 +23,7 @@ class HLL::Compiler does HLL::Backend::Default {
         @!stages     := nqp::split(' ', 'start parse ast ' ~ $!backend.stages());
 
         # Command options and usage.
-        @!cmdoptions := nqp::split(' ', 'e=s help|h target=s trace|t=s encoding=s output|o=s source-name=s combine version|v show-config verbose-config|V stagestats=s? ll-exception rxtrace nqpevent=s profile=s? profile-compile=s? profile-filename=s profile-stage=s'
+        @!cmdoptions := nqp::split(' ', 'e=s help|h target=s trace|t=s encoding=s output|o=s source-name=s combine version|v show-config verbose-config|V stagestats=s? ll-exception rxtrace nqpevent=s profile=s? profile-compile=s? profile-filename=s profile-stage=s force-stdin-eval-mode=s'
 #?if js
         ~ ' substagestats beautify nqp-runtime=s perl6-runtime=s libpath=s shebang execname=s source-map'
 #?endif
@@ -300,7 +300,16 @@ class HLL::Compiler does HLL::Backend::Default {
                 elsif !@a {
                     # Is STDIN a TTY display? If so, start the REPL, otherwise, simply
                     # assume the program to eval is given on STDIN.
-                    $result := stdin().t()
+                    my $force := %adverbs<force-stdin-eval-mode>//'';
+                    my $wants-interactive := $force
+                        ?? $force eq 'interactive'
+                          ?? 1 !! $force eq 'non-interactive'
+                            ?? 0 !! self.panic(
+                                "Unknown STDIN eval mode '$force'. Valid values"
+                                ~ " are 'non-interactive' and 'interactive'"
+                            )
+                        !! stdin().t();
+                    $result := $wants-interactive
                         ?? self.interactive(|%adverbs)
                         !! self.evalfiles('-', |%adverbs);
                 }