add .h file generation tests back

[andrewrk]

Extracted from #14647.

 ctx.h("simple header", linux_x64,
 \\export fn start() void{}
 ,
 \\zig_extern void start(void);
 \\
 );
 ctx.h("header with single param function", linux_x64,
 \\export fn start(a: u8) void{
 \\ _ = a;
 \\}
 ,
 \\zig_extern void start(uint8_t const a0);
 \\
 );
 ctx.h("header with multiple param function", linux_x64,
 \\export fn start(a: u8, b: u8, c: u8) void{
 \\ _ = a; _ = b; _ = c;
 \\}
 ,
 \\zig_extern void start(uint8_t const a0, uint8_t const a1, uint8_t const a2);
 \\
 );
 ctx.h("header with u32 param function", linux_x64,
 \\export fn start(a: u32) void{ _ = a; }
 ,
 \\zig_extern void start(uint32_t const a0);
 \\
 );
 ctx.h("header with usize param function", linux_x64,
 \\export fn start(a: usize) void{ _ = a; }
 ,
 \\zig_extern void start(uintptr_t const a0);
 \\
 );
 ctx.h("header with bool param function", linux_x64,
 \\export fn start(a: bool) void{_ = a;}
 ,
 \\zig_extern void start(bool const a0);
 \\
 );
 ctx.h("header with noreturn function", linux_x64,
 \\export fn start() noreturn {
 \\ unreachable;
 \\}
 ,
 \\zig_extern zig_noreturn void start(void);
 \\
 );
 ctx.h("header with multiple functions", linux_x64,
 \\export fn a() void{}
 \\export fn b() void{}
 \\export fn c() void{}
 ,
 \\zig_extern void a(void);
 \\zig_extern void b(void);
 \\zig_extern void c(void);
 \\
 );
 ctx.h("header with multiple includes", linux_x64,
 \\export fn start(a: u32, b: usize) void{ _ = a; _ = b; }
 ,
 \\zig_extern void start(uint32_t const a0, uintptr_t const a1);
 \\
 );

[WolfEYc]

My library generates a few hundred c export fns at comptime. Lookin forward 2 this one 4 sure!