`sigpipe` standalone test should be restored

[alexrp]

zig/test/standalone/build.zig.zon

Lines 145 to 149 in b5ec75b

	// TODO This test is disabled for doing naughty things in the build script.
	// The logic needs to get moved to a child process instead of build.zig.
	//.sigpipe = .{
	// .path = "sigpipe",
	//},

zig/test/standalone/sigpipe/build.zig

Lines 8 to 10 in b5ec75b

	// TODO signal handling code has no business being in a build script.
	// this logic needs to move to a file called parent.zig which is
	// added as an executable.

[rootbeer]

I think this test (or something with similar coverage at least), could migrate into the newish test/standalone/posix tests. I was hoping to get around to that at some point. But nuking the test in the meantime is fine (it was probably going to need to be re-written anyway).