Fix test "yaml is installed for python" in MacOS

plugins/python-build/bin/python-build

@@ -141,8 +141,9 @@ can_use_homebrew() {
   local brew_prefix
   command -v brew &>/dev/null && \
     # tests can have non-functional `brew' stub aliased to `false'
-    brew_prefix="$(brew --prefix)" && 
-    [[ $(abs_dirname "${BASH_SOURCE}") == "$(abs_dirname "${brew_prefix}")"/* ]] &&
+    # in Bash 3.2, var="$(cmd)" errexits on failure even if part of a conditional chain
+    brew_prefix="$(brew --prefix || true)" && [[ -n "$brew_prefix" ]] && \
+    [[ $(abs_dirname "${BASH_SOURCE}") == "$(abs_dirname "${brew_prefix}")"/* ]] && \
   { lock_in homebrew; return 0; }
   
   # do not check the same stuff multiple times

plugins/python-build/test/build.bats

@@ -70,6 +70,9 @@ assert_build_log() {
   install_fixture definitions/needs-yaml
   assert_success
 
+  unstub uname
+  unstub make
+
   assert_build_log <<OUT
 yaml-0.1.6: CFLAGS="" CPPFLAGS="-I${TMP}/install/include" LDFLAGS="-L${TMP}/install/lib -Wl,-rpath,${TMP}/install/lib" PKG_CONFIG_PATH=""
 yaml-0.1.6: --prefix=$INSTALL_ROOT
@@ -80,9 +83,6 @@ Python-3.6.2: --prefix=$INSTALL_ROOT --enable-shared --libdir=$INSTALL_ROOT/lib
 make -j 2
 make install
 OUT
-
-  unstub uname
-  unstub make
 }
 
 @test "apply global and package-specific flags, package flags come later to have precedence" {