No spaces around = in variable assignment

BSD make takes spaces around = literally
and produces a "TESTING_TOR_BINARY "
variable with a trailing space, which leads
to test_keygen.sh failure.

Fixes 17154

src/or/include.am

@@ -123,9 +123,9 @@ src_or_tor_cov_LDADD = src/or/libtor-testing.a src/common/libor-testing.a \
 	src/common/libor-event-testing.a src/trunnel/libor-trunnel-testing.a \
 	@TOR_ZLIB_LIBS@ @TOR_LIB_MATH@ @TOR_LIBEVENT_LIBS@ @TOR_OPENSSL_LIBS@ \
 	@TOR_LIB_WS32@ @TOR_LIB_GDI@ @CURVE25519_LIBS@ @TOR_SYSTEMD_LIBS@
-export TESTING_TOR_BINARY = $(top_builddir)/src/or/tor-cov
+export TESTING_TOR_BINARY=$(top_builddir)/src/or/tor-cov
 else
-export TESTING_TOR_BINARY = $(top_builddir)/src/or/tor
+export TESTING_TOR_BINARY=$(top_builddir)/src/or/tor
 endif
 
 ORHEADERS = \