Fix TAP::Harness choking on single backslashes in descriptions

Currently, TAP::Harness assumes backslashes in test descriptions will always be
either doubled, or followed by '#'. Since backslashing every slash produces
altered descriptions, we made Test.pm6 only backslash '#' in descriptions[^1],
but that broke the harness's expectations.

Fix by changing the grammar for description token to accept single slashes too.

Neither v12[^2], v13[^3], or v14 draft [^4] versions of TAP spec say
anything about backslashes or whether they can escape '#' in test
descriptions to prevent it from specifying a TAP directive. So the
correctitude of this commit's behaviour is based on what Perl 5's is doing.

P.S.: I couldn't figure out how to make TAP::Harness keep quiet and not produce
any output, hence such a weird test included in this work.

[1] b183cab
[2] https://testanything.org/tap-specification.html
[3] https://testanything.org/tap-version-13-specification.html
[4] TestAnything/testanything.github.io#36

lib/TAP.pm6

@@ -173,7 +173,7 @@ grammar Grammar {
         '1..' <count=.num> <.sp>* [ '#' <.sp>* $<directive>=[:i 'SKIP'] <.alnum>* [ <.sp>+ $<explanation>=[\N*] ]? ]?
     }
     regex description {
-        [ <-[\n\#\\]> | \\<[\\#]> ]+ <!after <sp>+>
+        [ '\\\\' || '\#' || <-[\n#]> ]+ <!after <sp>+>
     }
     token test {
         $<nok>=['not '?] 'ok' [ <.sp> <num> ]? ' -'?

t/06-tap-harness/01-desc-backslashes.t

@@ -0,0 +1,45 @@
+use v6;
+use lib <lib>;
+use Test;
+use TAP:auth<perl>;
+
+plan 8;
+
+constant $TEST_FILE
+= 't/06-tap-harness/test-files/01-desc-backslashes--1.TAP-TEST';
+
+# stupid hack to avoid TAP::Harness's output from interfering with our tests
+# I can't find docs for it to know how to do this properly; so run it as
+# as a separate script, make it do tests, then test that output with like()
+given run :out, :err, $*EXECUTABLE, '-e', ｢
+    use lib <lib>;
+    use Test;
+    use TAP:auth<perl>;
+
+    plan 8;
+
+    my $h = TAP::Harness.new;
+    my $w = $h.run('｣ ~ $TEST_FILE ~ ｢').waiter;
+    await $w;
+    given $w.result {
+        is .tests-planned,  12,    'HTEST1 planned';
+        is .passed,         12,    'HTEST2 passed';
+        is .failed,         0,     'HTEST3 failed';
+        is .errors,         0,     'HTEST4 errors';
+        is .skipped,        1,     'HTEST5 skipped';
+        is .todo,           0,     'HTEST6 todo';
+        is .todo-passed,    0,     'HTEST7 todo passed';
+        is .exit-failed,    False, 'HTEST8 exit failed';
+    }
+｣ {
+    given .err.slurp-rest { when :so { diag "STDERR output was ```$_```" } }
+
+    like $output, /^^ 'ok' \N+ 'HTEST1'/, 'planned';
+    like $output, /^^ 'ok' \N+ 'HTEST2'/, 'passed';
+    like $output, /^^ 'ok' \N+ 'HTEST3'/, 'failed';
+    like $output, /^^ 'ok' \N+ 'HTEST4'/, 'errors';
+    like $output, /^^ 'ok' \N+ 'HTEST5'/, 'skipped';
+    like $output, /^^ 'ok' \N+ 'HTEST6'/, 'todo';
+    like $output, /^^ 'ok' \N+ 'HTEST7'/, 'todo passed';
+    like $output, /^^ 'ok' \N+ 'HTEST8'/, '.exit-failed';
+}

t/06-tap-harness/test-files/01-desc-backslashes--1.TAP-TEST

@@ -0,0 +1,17 @@
+#!/usr/bin/env perl6
+use v6;
+use Test;
+plan 12;
+
+ok 1, 'fake todo # TODO it is fake 1';
+ok 1, 'fake todo \# TODO it is fake 2';
+ok 1, 'fake todo \\# TODO it is fake 3';
+ok 1, 'fake todo \\# TODO it is fake 4';
+ok 1, 'fake todo \\\# TODO it is fake 5';
+ok 1, 'fake todo \\\\# TODO it is fake 6';
+ok 1, 'fake todo \\\\\# TODO it is fake 7';
+ok 1, '\\';
+ok 1, '\\\\';
+ok 1, '\\\\\\';
+ok 1, '\\\\\\\\';
+ok 1, '\\\\\\\\\\';