Add a rudimentary testsuite.

This adds a very primitive test harness and tests relevant to the recent
changes to how program startup works, as well as the upcoming changes
to support LTO.

Author: sunfishcode

tests/.gitignore

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+*.observed
+*.wasm

tests/compile-only/addresses.c

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+#include <stdio.h>
+#include <limits.h>
+
+extern void __dso_handle;
+extern void __data_end;
+extern void __global_base;
+extern void __heap_base;
+
+int main(int argc, char *argv[]) {
+    printf("NULL=%p\n", NULL);
+    printf("__dso_handle=%p\n", &__dso_handle);
+    printf("__data_end=%p\n", &__data_end);
+    printf("__global_base=%p\n", &__global_base);
+    printf("__heap_base=%p\n", &__heap_base);
+    printf("__builtin_frame_address(0)=%p\n", __builtin_frame_address(0));
+    printf("__builtin_alloca(0)=%p\n", __builtin_alloca(0));
+    printf("__builtin_wasm_memory_size(0)=%p\n", (void *)(__builtin_wasm_memory_size(0) * PAGE_SIZE));
+    return 0;
+}

tests/compile-only/test.cc

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+int main(){}

tests/empty-expected.wat

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+(module
+  (type (;0;) (func))
+  (type (;1;) (func (result i32)))
+  (func $__wasm_call_ctors (type 0))
+  (func $_start (type 0)
+    call $__wasm_call_ctors)
+  (func $__main_void (type 1) (result i32)
+    i32.const 0)
+  (table (;0;) 1 1 funcref)
+  (memory (;0;) 2)
+  (global (;0;) (mut i32) (i32.const 66560))
+  (export "memory" (memory 0))
+  (export "_start" (func $_start)))

tests/exit_status_zero

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+0

tests/general/argc_argv_main.c

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+#include <stdio.h>
+
+int main(int argc, char *argv[]) {
+    puts("hello from argc argv main!");
+    return 0;
+}

tests/general/argc_argv_main.c.stdout.expected

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+hello from argc argv main!

tests/general/argc_argv_main.cc

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+#include <stdio.h>
+
+int main(int argc, char *argv[]) {
+    puts("hello from C++ argc argv main!");
+    return 0;
+}

tests/general/argc_argv_main.cc.stdout.expected

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+hello from C++ argc argv main!

tests/general/ctors_dtors.c

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+#include <stdlib.h>
+#include <stdio.h>
+#include <assert.h>
+#include <errno.h>
+extern char **environ;
+
+static void from_atexit(void) {
+    printf("hello from_atexit\n");
+}
+
+static void another_from_atexit(void) {
+    printf("hello another_from_atexit\n");
+}
+
+__attribute__((constructor)) static void from_constructor(void) {
+    printf("hello from_constructor\n");
+}
+
+__attribute__((constructor(101))) static void from_constructor_101(void) {
+    assert(errno == 0);
+    printf("hello from_constructor101\n");
+
+    assert(environ && "environment should be initialized by this point");
+}
+
+__attribute__((constructor(65535))) static void from_constructor_65535(void) {
+    printf("hello from_constructor65535\n");
+}
+
+__attribute__((destructor)) static void from_destructor(void) {
+    printf("hello from_destructor\n");
+}
+
+__attribute__((destructor(101))) static void from_destructor101(void) {
+    printf("hello from_destructor101\n");
+}
+
+__attribute__((destructor(65535))) static void from_destructor65535(void) {
+    printf("hello from_destructor65535\n");
+}
+
+int main(int argc, char *argv[]) {
+    printf("hello main\n");
+    assert(argc != 0);
+    assert(argv != NULL);
+    assert(argv[argc] == NULL);
+
+    atexit(from_atexit);
+    atexit(another_from_atexit);
+    printf("goodbye main\n");
+    return 0;
+}