stage2 test harness: show source file name in compile errors

[andrewrk]

Currently, compile errors in test cases look like this:

        case.addError("", &[_][]const u8{
            ":84:9: error: struct 'test_case.test_case' has no member named 'main'",
        });

The source file name is not included. Similarly, if you get a compile error, it gets truncated in the message:

[nix-shell:~/dev/zig/build-release]$ ./zig build test-stage2 -Denable-qemu -Denable-wasmtime -Denable-llvm 
tests [1/78] CBE: hello world with updates [1/4] update [3/3] 
Case 'CBE: hello world with updates': unexpected errors at update_index=0:
================================================================================
:20:81: error: redundant comptime keyword in already comptime scope
================================================================================
test 'CBE: hello world with updates' failed: UnexpectedCompileErrors

tests [2/78] CBE: var args [1/1] update [3/3] 
Case 'CBE: var args': unexpected errors at update_index=0:
================================================================================
:20:81: error: redundant comptime keyword in already comptime scope
================================================================================
test 'CBE: var args' failed: UnexpectedCompileErrors

tests [3/78] CBE: @intToError [1/3] update [3/3] 
Case 'CBE: @intToError': unexpected errors at update_index=0:
================================================================================
:20:81: error: redundant comptime keyword in already comptime scope
================================================================================
test 'CBE: @intToError' failed: UnexpectedCompileErrors

tests [4/78] CBE: x86_64-linux inline assembly [1/5] update [3/3] 
Case 'CBE: x86_64-linux inline assembly': unexpected errors at update_index=0:
================================================================================
:20:81: error: redundant comptime keyword in already comptime scope
================================================================================
test 'CBE: x86_64-linux inline assembly' failed: UnexpectedCompileErrors

tests [5/78] CBE: alloc and retptr [1/1] update [3/3] 
Case 'CBE: alloc and retptr': unexpected errors at update_index=0:
================================================================================
:20:81: error: redundant comptime keyword in already comptime scope
================================================================================
test 'CBE: alloc and retptr' failed: UnexpectedCompileErrors


...and so on.

Here the change that I made was:

--- a/lib/std/crypto/md5.zig
+++ b/lib/std/crypto/md5.zig
@@ -17,7 +17,7 @@ const RoundParam = struct {
     t: u32,
 };
 
-fn roundParam(a: usize, b: usize, c: usize, d: usize, k: usize, s: u32, t: u32) RoundParam {
+fn roundParam(a: usize, b: usize, c: usize, d: usize, k: usize, s: u32, t: u32) comptime RoundParam {
     return RoundParam{
         .a = a,
         .b = b,

But the compile error did not show md5.zig which would have saved a lot of time. This is now important, because AstGen compile errors can now fail the CI for std lib contributions!

Related: #8828