Add explicit Print::write(char) (#6101)

W/o this change, file::write('a'); tries to use the template and fails
since char is a basic type.

The reason it is needed is due to pre 2.5.x behavior, File::write(char)
silently was cast to File::write(uint8_t).  With the template write,
though, this is not performed.

* Add Print::write tests and add'l overrides

Ensure that print::write does something sane and doesn't cause a compile
time error about templates when used for Files.

Test using SPIFFS file since Print is an abstract type.

tests/host/Makefile

@@ -127,7 +127,8 @@ TEST_CPP_FILES := \
 	core/test_pgmspace.cpp \
 	core/test_md5builder.cpp \
 	core/test_string.cpp \
-	core/test_PolledTimeout.cpp
+	core/test_PolledTimeout.cpp \
+	core/test_Print.cpp
 
 PREINCLUDES := \
 	-include common/mock.h \