build: Fix Makefile wrapper to properly quote test regex (#5146)

This fixes a longstanding issue where saying

    make test TEST="foo|bar"

would be malformed, instead of running tests that matched either foo or
bar (misinterpreting the `|` as a pipe). The problem was just not
quoting it properly.

Signed-off-by: Larry Gritz <lg@larrygritz.com>

Makefile

@@ -159,7 +159,7 @@ MY_CMAKE_FLAGS += -DTEX_BATCH_SIZE:STRING="${TEX_BATCH_SIZE}"
 endif
 
 ifneq (${TEST},)
-TEST_FLAGS += -R ${TEST}
+TEST_FLAGS += -R '${TEST}'
 endif
 
 ifneq (${USE_CCACHE},)