[tests] New assert function assume with test message first

* Motivation

Introduce single assert function which can:
- use emulated named arguments
- evaluate code to build "got" value
- detect eval success/failure
- detect expected eval success/failure

`assume` function enforces most important part of test case,
assumed behaviour description (test message), first.

Author: Branislav Zahradník

t/test.pl

@@ -41,6 +41,11 @@
 our $NO_ENDING = 0;
 our $Tests_Are_Passing = 1;
 
+sub diag;
+sub fail;
+sub is;
+sub like;
+
 # Use this instead of print to avoid interference while testing globals.
 sub _print {
     local($\, $", $,) = (undef, ' ', '');
@@ -52,6 +57,49 @@ sub _print_stderr {
     print STDERR @_;
 }
 
+sub assume {
+	my ($message, %args) = @_;
+
+	my $got;
+	my $lives = eval qq {
+		do {
+			use strict;
+			use warnings;
+			my \$result = q ();
+			\$got = do { $args{eval}; };
+		};
+		1;
+	};
+	my $error = $@;
+
+	if (exists $args{throws}) {
+		if ($lives) {
+			my $rv = fail $message;
+			diag q (Expected to fail but it lives);
+            return $rv;
+		}
+
+		return ref $args{throws}
+			? like $@, $args{throws}, $message
+			: is   $@, $args{throws}, $message
+			;
+	}
+
+	unless ($lives) {
+		my $rv = fail $message;
+
+        diag q (Expected to live but it died:);
+        diag $error =~ s (^) (  )rmg;
+
+        return $rv;
+	}
+
+	return ref $args{expect}
+		? like $got, $args{expect}, $message
+		: is   $got, $args{expect}, $message
+		;
+}
+
 sub plan {
     my $n;
     if (@_ == 1) {