test: try to guess the build directory

This will work if $(TOP_SRC_DIR) has exactly one subdirectory with
.ninja_deps. Otherwise, BUILD_DIR has to be specified.

test/Makefile.guess

@@ -0,0 +1,14 @@
+# Try to guess the build directory:
+# we look for subdirectories of ../.. that look like ninja build dirs.
+
+ifeq ($(BUILD_DIR),)
+        dirs = $(dir $(wildcard ../../*/.ninja_log))
+        ifeq ($(dirs),)
+                $(error Cannot guess build dir, set BUILD_DIR)
+        endif
+        ifneq ($(firstword $(dirs)),$(dirs))
+                $(warning Candidates: $(dirs))
+                $(error Too many build dirs to pick from, set BUILD_DIR)
+        endif
+        BUILD_DIR=$(dirs)
+endif

test/TEST-01-BASIC/Makefile

@@ -1,2 +1,4 @@
+include ../Makefile.guess
+
 all setup clean run:
-	@basedir=../.. TEST_BASE_DIR=../ ./test.sh --$@
+	@basedir=../.. TEST_BASE_DIR=../ BUILD_DIR=$(BUILD_DIR) ./test.sh --$@

test/TEST-13-NSPAWN-SMOKE/Makefile

@@ -1,5 +1,7 @@
+include ../Makefile.guess
+
 all setup run:
-	@basedir=../.. TEST_BASE_DIR=../ ./test.sh --$@
+	@basedir=../.. TEST_BASE_DIR=../ BUILD_DIR=$(BUILD_DIR) ./test.sh --$@
 clean:
 	@basedir=../.. TEST_BASE_DIR=../ ./test.sh --clean
 	@rm -f has-overflow